Bug #1719995: don't use deprecated method in sets example.

 (backport from rev. 55383)

Doc/lib/libsets.tex

@@ -189,7 +189,7 @@ backwards compatibility.  Programmers should prefer the
 Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
 >>> employees.issuperset(engineers)     # superset test
 False
->>> employees.union_update(engineers)         # update from another set
+>>> employees.update(engineers)         # update from another set
 >>> employees.issuperset(engineers)
 True
 >>> for group in [engineers, programmers, managers, employees]: