add extra var for local variable

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

@@ -1073,7 +1073,7 @@ define([
      * @param {Number} [level] A heading level (e.g., 1 becomes <h1>)
      */
     Notebook.prototype.to_heading = function (index, level) {
-        level = level || 1;
+        var level = level || 1;
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_element = this.get_cell_element(i);