forgotten selected_cell -> get_selected_cell

somwhere in renaming `selected_cell` to `get_selected_cell` one
occurence has been forgoten in `codecell.js` this was mainly visible by
the fact that the 'show pager' button of the tooltip was non fonctionnal
anymore.

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

@@ -224,7 +224,7 b' var IPython = (function (IPython) {'
         morelink.append(morespan);
         morelink.click(function(){
             var msg_id = IPython.notebook.kernel.execute(name+"?");
-            IPython.notebook.msg_cell_map[msg_id] = IPython.notebook.selected_cell().cell_id;
+            IPython.notebook.msg_cell_map[msg_id] = IPython.notebook.get_selected_cell().cell_id;
             that.remove_and_cancel_tooltip();
             setTimeout(function(){that.code_mirror.focus();}, 50);
         });