[docs] fix github PR links in changelog
[docs] fix github PR links in changelog

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