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

diff --git a/Lib/enum.py b/Lib/enum.py
index 0c6d8c1..49c46ea 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -79,7 +79,7 @@
     """
     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)