examples: improve interceptor example with better markdown formatting (#8612)

This PR changes the existing example to use fenced code blocks instead
of backticks to show the interceptor signatures. This greatly improves
how the document is rendered. The PR also changes the word `overload` to
`override` because it is the latter that we are showcasing here, and Go
does not support overloading anyways.

RELEASE NOTES: none

---------

Co-authored-by: Doug Fawley <[email protected]>

Author: easwars

examples/features/interceptor/README.md

@@ -30,11 +30,14 @@ types of interceptors in total.
 
 The type for client-side unary interceptors is
 [`UnaryClientInterceptor`](https://godoc.org/google.golang.org/grpc#UnaryClientInterceptor).
-It is essentially a function type with signature: `func(ctx context.Context,
-method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker,
-opts ...CallOption) error`. Unary interceptor implementations can usually be
-divided into three parts: pre-processing, invoking the RPC method, and
-post-processing.
+It is a function type with signature:
+
+```golang
+func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error
+```
+
+Unary interceptor implementations can usually be divided into three parts:
+pre-processing, invoking the RPC method, and post-processing.
 
 For pre-processing, users can get info about the current RPC call by examining
 the args passed in. The args include the RPC context, method string, request to
@@ -59,22 +62,26 @@ To install a unary interceptor on a ClientConn, configure `Dial` with the
 
 The type for client-side stream interceptors is
 [`StreamClientInterceptor`](https://godoc.org/google.golang.org/grpc#StreamClientInterceptor).
-It is a function type with signature: `func(ctx context.Context, desc
-*StreamDesc, cc *ClientConn, method string, streamer Streamer, opts
-...CallOption) (ClientStream, error)`. An implementation of a stream interceptor
-usually includes pre-processing, and stream operation interception.
+It is a function type with signature:
+
+```golang
+func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, streamer Streamer, opts ...CallOption) (ClientStream, error)
+```
+
+An implementation of a stream interceptor usually includes pre-processing, and
+stream operation interception.
 
 The pre-processing is similar to unary interceptors.
 
 However, rather than invoking the RPC method followed by post-processing, stream
 interceptors intercept the users' operations on the stream. The interceptor
 first calls the passed-in `streamer` to get a `ClientStream`, and then wraps the
-`ClientStream` while overloading its methods with the interception logic.
+`ClientStream` while overriding its methods with the interception logic.
 Finally, the interceptor returns the wrapped `ClientStream` to user to operate
 on.
 
 In the example, we define a new struct `wrappedStream`, which embeds a
-`ClientStream`. We then implement (overload) the `SendMsg` and `RecvMsg` methods
+`ClientStream`. We then implement (override) the `SendMsg` and `RecvMsg` methods
 on `wrappedStream` to intercept these two operations on the embedded
 `ClientStream`. In the example, we log the message type info and time info for
 interception purpose.
@@ -92,9 +99,11 @@ different information provided as args.
 
 The type for server-side unary interceptors is
 [`UnaryServerInterceptor`](https://godoc.org/google.golang.org/grpc#UnaryServerInterceptor).
-It is a function type with signature: `func(ctx context.Context, req
-interface{}, info *UnaryServerInfo, handler UnaryHandler) (resp interface{}, err
-error)`.
+It is a function type with signature:
+
+```golang
+func(ctx context.Context, req interface{}, info *UnaryServerInfo, handler UnaryHandler) (resp interface{}, err error)
+```
 
 Refer to the client-side unary interceptor section for a detailed implementation
 and explanation.
@@ -107,13 +116,15 @@ To install a unary interceptor on a Server, configure `NewServer` with the
 
 The type for server-side stream interceptors is
 [`StreamServerInterceptor`](https://godoc.org/google.golang.org/grpc#StreamServerInterceptor).
-It is a function type with the signature: `func(srv interface{}, ss
-ServerStream, info *StreamServerInfo, handler StreamHandler) error`.
+It is a function type with the signature:
+
+```golang
+func(srv interface{}, ss ServerStream, info *StreamServerInfo, handler StreamHandler) error
+```
 
 Refer to the client-side stream interceptor section for a detailed
 implementation and explanation.
 
 To install a stream interceptor on a Server, configure `NewServer` with the
 [`StreamInterceptor`](https://godoc.org/google.golang.org/grpc#StreamInterceptor)
 `ServerOption`.
-