fix use of undefined markup

Doc/api/concrete.tex

@@ -98,7 +98,7 @@ There is no \cfunction{PyNone_Check()} function for the same reason.
 \end{cvardesc}
 
 \begin{csimplemacrodesc}{Py_RETURN_NONE}
-  Properly handles returning \cvar{Py_None} from within a C function.
+  Properly handles returning \cdata{Py_None} from within a C function.
 \end{csimplemacrodesc}