use modal_stretch on keyboard shortcut dialog

IPython/html/static/notebook/js/quickhelp.js

@@ -62,6 +62,7 b' var IPython = (function (IPython) {'
                 Close : {}
             }
         });
+        this.shortcut_dialog.addClass("modal_stretch");
         
         $([IPython.events]).on('rebuild.QuickHelp', function() { that.force_rebuild = true;});
     };