Document the correct return type of PyLong_AsUnsignedLongLongMask.

Doc/api/concrete.tex

@@ -376,7 +376,7 @@ booleans.  The following macros are available, however.
   \versionadded{2.3}
 \end{cfuncdesc}
 
-\begin{cfuncdesc}{unsigned long}{PyLong_AsUnsignedLongLongMask}{PyObject *io}
+\begin{cfuncdesc}{unsigned PY_LONG_LONG}{PyLong_AsUnsignedLongLongMask}{PyObject *io}
   Return a C \ctype{unsigned long long} from a Python long integer, without
   checking for overflow.
   \versionadded{2.3}