doc: fix typo in contribute.html
doc: fix typo in contribute.html

Change-Id: Ica27c4a9e4c364d94250aebfc4c2b59cff7f4a8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/258679
Trust: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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