doc/codewalk: make it work with jQuery 1.8.2

    so that it could work with the bundled jQuery.

R=adg
CC=golang-dev
https://golang.org/cl/6592073

doc/codewalk/codewalk.js

@@ -297,7 +297,7 @@ CodewalkViewer.prototype.updateHeight = function() {
 };
 
 jQuery(document).ready(function() {
-  var viewer = new CodewalkViewer(jQuery());
+  var viewer = new CodewalkViewer(jQuery('#codewalk-main'));
   viewer.selectFirstComment();
   viewer.targetCommentLinksAtBlank();
   viewer.installEventHandlers();