Fix duplicate target.

Doc/faq/extending.rst

@@ -48,7 +48,7 @@ Python's C API.
 If you need to interface to some C or C++ library for which no Python extension
 currently exists, you can try wrapping the library's data types and functions
 with a tool such as `SWIG <http://www.swig.org>`_.  `SIP
-<http://www.riverbankcomputing.co.uk/software/sip/>`_, `CXX
+<http://www.riverbankcomputing.co.uk/software/sip/>`__, `CXX
 <http://cxx.sourceforge.net/>`_ `Boost
 <http://www.boost.org/libs/python/doc/index.html>`_, or `Weave
 <http://www.scipy.org/site_content/weave>`_ are also alternatives for wrapping