[release-branch.go1.9] doc/contrib.html: fix outdated link

Change-Id: I6d94a14a781a2fbeb90db35ae3490ce2ac5f8ed1
Reviewed-on: https://go-review.googlesource.com/83315
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 2fb9fe48c63ed0b7eee36d0d8c2d494ad70602b0)
Reviewed-on: https://go-review.googlesource.com/89036
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/doc/contrib.html b/doc/contrib.html
index 0290923..c165f39 100644
--- a/doc/contrib.html
+++ b/doc/contrib.html
@@ -117,6 +117,6 @@
 <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>