doc: fix missing anchor links in contribute.html

Some <h3>s were missing ids due to which the anchor links
weren't getting generated.

Fixes #32415

Change-Id: Ica21425c1a7c49735231c1de96b6c77dd594ce64
Reviewed-on: https://go-review.googlesource.com/c/go/+/180397Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -26,7 +26,7 @@ see <a href="gccgo_contribute.html">Contributing to gccgo</a>.
 
 <h2 id="contributor">Becoming a contributor</h2>
 
-<h3>Overview</h3>
+<h3 id="contrib_overview">Overview</h3>
 
 <p>
 The first step is registering as a Go contributor and configuring your environment.
@@ -261,7 +261,7 @@ a new issue</a> or by claiming
 an <a href="https://golang.org/issues">existing one</a>.
 </p>
 
-<h3>Check the issue tracker</h3>
+<h3 id="check_tracker">Check the issue tracker</h3>
 
 <p>
 Whether you already know what contribution to make, or you are searching for
@@ -398,7 +398,7 @@ It's different but powerful and familiarity with it will help you understand
 the flow.
 </p>
 
-<h3>Overview</h3>
+<h3 id="gerrit_overview">Overview</h3>
 
 <p>
 This is an overview of the overall process:
@@ -666,7 +666,7 @@ The algorithm is described at https://wikipedia.org/wiki/McGillicutty_Algorithm
 Fixes #159
 </pre>
 
-<h3>First line</h3>
+<h3 id="first_line">First line</h3>
 
 <p>
 The first line of the change description is conventionally a short one-line
@@ -684,7 +684,7 @@ and actually summarizes the result of the change.
 Follow the first line by a blank line.
 </p>
 
-<h3>Main content</h3>
+<h3 id="main_content">Main content</h3>
 
 <p>
 The rest of the description elaborates and should provide context for the
@@ -702,7 +702,7 @@ tool is conventionally used to format
 benchmark data for change descriptions.
 </p>
 
-<h3>Referencing issues</h3>
+<h3 id="ref_issues">Referencing issues</h3>
 
 <p>
 The special notation "Fixes #12345" associates the change with issue 12345 in the