Update notebook.rst

Updating broken reference to notebook examples 

Since the notebook directory doesn't contain a readme anymore, updating to point to the index notebook rendered via nbviewer

docs/source/notebook/notebook.rst

@@ -204,7 +204,7 b' toolbar (which will be "Code", initially), or via :ref:`keyboard shortcuts'
 
 For more information on the different things you can do in a notebook,
 see the `collection of examples
-<https://github.com/ipython/ipython/tree/master/examples/notebooks#readme>`_.
+<http://nbviewer.ipython.org/github/ipython/ipython/blob/master/examples/Notebook/Index.ipynb>`_.
 
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