Fix a typo in _make_class_unpicklable() docstring (GH-26729)

(cherry picked from commit 689a84475e7b1da79d5ae82df67ab8897316f98c)

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>
diff --git a/Lib/enum.py b/Lib/enum.py
index bf3460c..9f9c89e 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -79,7 +79,7 @@ def _make_class_unpicklable(obj):
     """
     Make the given obj un-picklable.
 
-    obj should be either a dictionary, on an Enum
+    obj should be either a dictionary, or an Enum
     """
     def _break_on_call_reduce(self, proto):
         raise TypeError('%r cannot be pickled' % self)