doc/contrib.html: fix outdated link

Change-Id: I6d94a14a781a2fbeb90db35ae3490ce2ac5f8ed1
Reviewed-on: https://go-review.googlesource.com/83315
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Stanislav Afanasev

doc/contrib.html

@@ -118,6 +118,6 @@ <h3><a href="/doc/contribute.html">Contributing code</a></h3>
 <p>
 Check <a href="//golang.org/issue">the tracker</a> for 
 open issues that interest you. Those labeled
-<a href="https://github.com/golang/go/issues?q=is%3Aopen+is%3Aissue+label%3Ahelpwanted">helpwanted</a>
+<a href="https://github.com/golang/go/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22">help wanted</a>
 are particularly in need of outside help.
 </p>