Bug #1719995: don't use deprecated method in sets example.
(backport from rev. 55383)
diff --git a/Doc/lib/libsets.tex b/Doc/lib/libsets.tex
index 22bf34b..fbf03c8 100644
--- a/Doc/lib/libsets.tex
+++ b/Doc/lib/libsets.tex
@@ -187,13 +187,13 @@
>>> engineers.add('Marvin') # add element
>>> print engineers
Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
->>> employees.issuperset(engineers) # superset test
+>>> 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]:
-... group.discard('Susan') # unconditionally remove element
+... group.discard('Susan') # unconditionally remove element
... print group
...
Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])