godocs.js: fix bad variable name

Thanks to anno.langen.
Fixes #103.

R=r
https://golang.org/cl/155050

doc/godocs.js

@@ -13,7 +13,7 @@
  */
 if (window.addEventListener){
   window.addEventListener('load', godocs_onload, false);
-} else if (elm.attachEvent){
+} else if (window.attachEvent){
   window.attachEvent('onload', godocs_onload);
 }