Fix 404 widget script URL

Fix 404 widget script URL to be compatible with HTTPS by replacing `http://` with `//`. The script hosted by Google is available on both protocols.

Author: Alfred Xing

404.html

@@ -151,7 +151,7 @@ <h1>Not found <span>:(</span></h1>
  <script>
  var GOOG_FIXURL_LANG = (navigator.language || '').slice(0,2),GOOG_FIXURL_SITE = location.host;
  </script>
- <script src="http://linkhelp.clients.google.com/tbproxy/lh/wm/fixurl.js"></script>
+ <script src="//linkhelp.clients.google.com/tbproxy/lh/wm/fixurl.js"></script>
  </div>
  </body>
 </html>