Point to the stable SymPy docs, not the dev docs

IPython/html/templates/notebook.html

@@ -234,7 +234,7 b' class="notebook_app"'
                             ("http://docs.scipy.org/doc/numpy/reference/","NumPy",True),
                             ("http://docs.scipy.org/doc/scipy/reference/","SciPy",True),
                             ("http://matplotlib.org/contents.html","Matplotlib",True),
-                            ("http://docs.sympy.org/dev/index.html","SymPy",True),
+                            ("http://docs.sympy.org/latest/index.html","SymPy",True),
                             ("http://pandas.pydata.org/pandas-docs/stable/","pandas", True)
                         )
                     )