correct mailing list URL

R=r
http://go/go-review/1024047

doc/contribute.html

@@ -14,7 +14,7 @@ see <a href="gccgo_contribute.html">Contributing to gccgo</a>.)
 <p>
 Before embarking on a significant change to an existing
 package or the creation of a major new package,
-it's a good idea to send mail to the <a href="FIXME">mailing list</a>
+it's a good idea to send mail to the <a href="http://groups.google.com/group/golang-nuts">mailing list</a>
 to let people know what you are thinking of doing.
 Doing so helps avoid duplication of effort and
 enables discussions about design before much code