Fix minor LaTeX bug of not ending a \NULL{} with the curly braces to get the

spacing correct in the output.

Doc/api/abstract.tex

@@ -831,7 +831,7 @@ determination.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{PyObject*}{PySequence_ITEM}{PyObject *o, int i}
-  Return the \var{i}th element of \var{o} or \NULL on failure.
+  Return the \var{i}th element of \var{o} or \NULL{} on failure.
   Macro form of \cfunction{PySequence_GetItem()} but without checking
   that \cfunction{PySequence_Check(\var{o})} is true and without
   adjustment for negative indices.