[release-branch.go1.12] 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 <bradfitz@golang.org>
(cherry picked from commit d0fadb93)
Reviewed-on: https://go-review.googlesource.com/c/go/+/173361


Reviewed-by: Andrew Bonventre <andybons@golang.org>