doc: update wording in contribution guide
doc: update wording in contribution guide

The top right menu in Gerrit is now a gear icon, and the link
has a slightly different title.

Change-Id: I3f5d194f31ad09a99416a45db392aa4b5c7d98ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/173400
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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