fixing a minor spelling mistake

Doc/lib/libstdtypes.tex

@@ -506,7 +506,7 @@ Notes:
 \item[(3)] Raises \exception{ValueError} when \var{x} is not found in
   \var{s}.
 
-\item[(4)] The \method{pop()} method only supported by the list and
+\item[(4)] The \method{pop()} method is only supported by the list and
   array types.  The optional argument \var{i} defaults to \code{-1},
   so that by default the last item is removed and returned.