doc: fix stale gofrontend/gccgo contribution link

Change-Id: I63af8f0a19ec91f4a2001aa7a2eadcd2232a47df
Reviewed-on: https://go-review.googlesource.com/28348Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/gccgo_contribute.html

@@ -12,7 +12,7 @@ information on building gccgo for yourself,
 see <a href="/doc/gccgo_install.html">Setting up and using gccgo</a>.
 For more of the gritty details on the process of doing development
 with the gccgo frontend,
-see <a href="https://code.google.com/p/gofrontend/source/browse/HACKING">the
+see <a href="https://go.googlesource.com/gofrontend/+/master/HACKING">the
 file HACKING</a> in the gofrontend repository.
 </p>