Fix include file reference in docs.

docs/src/userguide/language_basics.rst

@@ -291,7 +291,7 @@ about object parameters in C functions.
 To create a borrowed reference, specify the parameter type as ``PyObject*``.
 Cython won't perform automatic ``Py_INCREF``, or ``Py_DECREF``, e.g.:
 
-.. literalinclude:: ../../examples/userguide/language_basics/parameter_refcount.pxd
+.. literalinclude:: ../../examples/userguide/language_basics/parameter_refcount.pyx
 
 will display::