fix broken link

R=rsc, iant
CC=go-dev
http://go/go-review/1019003

doc/go_gccgo_setup.html

@@ -10,7 +10,7 @@ the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public
 License</a>.
 
 Note that <code>gccgo</code> is not the <code>6g</code> compiler; see
-the <a href="go_setup.html"><code>6g</code> set up</a> instructions
+the <a href="install.html"><code>6g</code> set up</a> instructions
 for that compiler.
 
 <h2 id="Source_code">Source code</h2>