doc/contribute: improve wording of Gerrit description

The word "of" was removed in https://go-review.googlesource.com/c/36626

Change-Id: Iece69f425d06ab1cf02743b1033cfed2e96667ab
Reviewed-on: https://go-review.googlesource.com/54290Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -158,7 +158,7 @@ completed and update the <code>AUTHORS</code> file.
 Changes to Go must be reviewed before they are accepted, no matter who makes the change.
 A custom git command called <code>git-codereview</code>, discussed below,
 helps manage the code review process through a Google-hosted
-<a href="https://go-review.googlesource.com/">instance</a> Gerrit.
+<a href="https://go-review.googlesource.com/">instance</a> of Gerrit.
 </p>
 
 <h3 id="git-codereview_install">Install the git-codereview command</h3>