Make consistent with 2.4

Doc/lib/libsets.tex

@@ -170,7 +170,7 @@ an argument.
 Also note, the module also includes a \method{union_update()} method
 which is an alias for \method{update()}.  The method is included for
 backwards compatibility.  Programmers should prefer the
-\method{update()} method because it is the one supported by the builtin
+\method{update()} method because it is supported by the builtin
 \class{set()} and \class{frozenset()} types.
 
 \subsection{Example \label{set-example}}