change --enable-languages flag to include c, c++ in first mention

R=iant
http://go/go-review/1026016

doc/gccgo_install.html

@@ -34,7 +34,7 @@ Building <code>gccgo</code> is just like building <code>gcc</code>
 with one additional option.  See
 the <a href="http://gcc.gnu.org/install/">instructions on the gcc web
 site</a>.  When you run <code>configure</code>, add the
-option <code>--enable-languages=go</code> (along with other languages you
+option <code>--enable-languages=c,c++,go</code> (along with other languages you
 may want to build).
 </p>