doc: fix bad link in go1.9 release notes

Change-Id: I64ba37428f5cc560f0f20fe039feaecf5fcda93e
Reviewed-on: https://go-review.googlesource.com/53330

Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -737,7 +737,7 @@ version of gccgo.
       <li><!-- CL 35488 -->
         The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
         now supports making requests via SOCKS5 proxy when the URL returned by
-        <a href="/net/http/#Transport.Proxy"><code>Transport.Proxy</code></a>
+        <a href="/pkg/net/http/#Transport.Proxy"><code>Transport.Proxy</code></a>
         has the scheme <code>socks5</code>.
       </li>
     </ul>