Pointing to the 2.x branch in the Help menu.

IPython/html/templates/notebook.html

@@ -226,7 +226,7 b' class="notebook_app"'
                     sections = (
                         (
                             ("http://ipython.org/documentation.html","IPython Help",True),
-                            ("http://nbviewer.ipython.org/github/ipython/ipython/tree/master/examples/Index.ipynb", "Notebook Help", True),
+                            ("http://nbviewer.ipython.org/github/ipython/ipython/tree/2.x/examples/Index.ipynb", "Notebook Help", True),
                         ),(
                             ("http://docs.python.org","Python",True),
                             ("http://docs.scipy.org/doc/numpy/reference/","NumPy",True),