[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 <[email protected]>
(cherry picked from commit d0fadb9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/173361
Reviewed-by: Andrew Bonventre <[email protected]>

Author: bsiegert

doc/contribute.html

@@ -46,7 +46,8 @@ <h3>Overview</h3>
 <li>
 <b>Step 2</b>: Configure authentication credentials for the Go Git repository.
 Visit <a href="https://go.googlesource.com/">go.googlesource.com</a>, click
-on "Generate Password" (top right), and follow the instructions.
+on the gear icon (top right), then on "Obtain password", and follow the
+instructions.
 </li>
 <li>
 <b>Step 3</b>: Register for Gerrit, the code review tool used by the Go team,