[docs] fix markdown link in docs/source/internals.rst
[docs] fix markdown link in docs/source/internals.rst

jcb91/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,juhasch/IPython-notebook-extensions,jcb91/IPython-notebook-extensions,juhasch/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions,juhasch/IPython-notebook-extensions,jcb91/IPython-notebook-extensions,ipython-contrib/IPython-notebook-extensions