fix reference to local copy of IPython license file

Cython/Build/IpythonMagic.py

@@ -41,7 +41,7 @@ Parts of this code were taken from Cython.inline.
 #
 # Distributed under the terms of the Modified BSD License.
 #
-# The full license is in the file COPYING.txt, distributed with this software.
+# The full license is in the file ipython-COPYING.rst, distributed with this software.
 #-----------------------------------------------------------------------------
 
 from __future__ import absolute_import, print_function