bpo-45391: mark UnionType as a class in documentation (GH-28757)


Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>
(cherry picked from commit e2cfc89e099b8fad5d8d5bd7f59dadffb6078778)

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>
diff --git a/Doc/library/types.rst b/Doc/library/types.rst
index 2314b02..5cd42f1 100644
--- a/Doc/library/types.rst
+++ b/Doc/library/types.rst
@@ -312,7 +312,7 @@
       This type can now be subclassed.
 
 
-.. data:: UnionType
+.. class:: UnionType
 
    The type of :ref:`union type expressions<types-union>`.