fix a typo introduced by a rebased PR

caused heading cells to appear in both rendered and unrendered state

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

@@ -531,7 +531,7 b' var IPython = (function (IPython) {'
                     .text('¶')
             );
             this.set_rendered(h);
-            this.element.find('div.text_cell_input').hide();
+            this.element.find('div.input_area').hide();
             this.element.find("div.text_cell_render").show();
             this.typeset();
         }