fix(grpcroute-doc): fix grpcroute structure definition error in the document

[0xff-dev]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
According to the definition of grpcroute v1, the matches[index].headers structure in the document is incorrectly defined.

In addition, should the structure definition here be adjusted to:

apiVersion: gateway.networking.k8s.io/v1
kind: GRPCRoute
...
spec:
  rules:
  - matches:
    - method:
        service: com.example.User
        method: Login
      headers:
      - name: version
        value: "2"
    - method:
        service: com.example.v2.User
        method: Login

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

None

[k8s-ci-robot]

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