[PR] Docs: Fix predicate functions anchor link

Summary:
Usually clicking the `#` symbol next to a heading in the docs links to that section. However this is broken for the "Predicate functions" section on https://flow.org/en/docs/types/functions/
Closes facebook#5783

Differential Revision: D7423666

Pulled By: mroch

fbshipit-source-id: c94d9fde40df8fde064dfaf2a06e339d028d8296

Author: sgrowe

website/en/docs/types/functions.md

@@ -213,7 +213,7 @@ var num: number = method.call(42);
 var str: string = method.call(42);
 ```
 
-### Predicate Functions <a class="toc" id="toc-function-checks" href="#toc-predicate-functions"></a>
+### Predicate Functions <a class="toc" id="toc-predicate-functions" href="#toc-predicate-functions"></a>
 
 Sometimes you will want to move the condition from an `if` statement into a function: