Fixing minor typo in menubar.js.

IPython/frontend/html/notebook/static/js/menubar.js

@@ -24,7 +24,7 b' var IPython = (function (IPython) {'
     MenuBar.prototype.style = function () {
         $('ul#menus').menubar({
             select : function (event, ui) {
-                // The selected cell looses focus when the menu is entered, so we
+                // The selected cell loses focus when the menu is entered, so we
                 // re-select it upon selection.
                 var i = IPython.notebook.selected_index();
                 IPython.notebook.select(i);