doc: fix bogus HTML in contribute.html

Introduced in https://golang.org/cl/20462

Change-Id: I6c55f87228f0980058a10f204b57ead7594e3492
Reviewed-on: https://go-review.googlesource.com/20490Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/22075Reviewed-by: Minux Ma <minux@golang.org>

doc/contribute.html

@@ -78,7 +78,7 @@ 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> of the code review
-system called <a https://www.gerritcodereview.com/">Gerrit</a>.
+system called <a href="https://www.gerritcodereview.com/">Gerrit</a>.
 </p>
 
 <h3 id="auth">Set up authentication for code review</h3>