Fix broken link.

Author: brianmay

docs/usage.rst

@@ -55,6 +55,8 @@ Additional information for TPROXY
 ---------------------------------
 TPROXY is the only method that supports full support of IPv6 and UDP.
 
+.. _PyXAPI: http://www.pps.univ-paris-diderot.fr/~ylg/PyXAPI/
+
 Full UDP or DNS support with the TPROXY method requires the ``recvmsg()``
 syscall. This is not available in Python 2, however is in Python 3.5 and
 later. Under Python 2 you might find it sufficient installing PyXAPI_ to get