doc: fix missing anchor links in contribute.html
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/+/180397
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,akutz/go,golang/go,nawawi/go,golang/go,akutz/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,CAFxX/go