[3.6] bpo-30619: Clarify typing.Union documentation (GH-2326) (GH-2337)

When a class and its subclass are present, the latter is skipped.
(cherry picked from commit 6580c19)

Author: Mariatta

Doc/library/typing.rst

@@ -939,7 +939,7 @@ The module defines the following classes, functions and decorators:
 
        Union[int, str] == Union[str, int]
 
-   * When a class and its subclass are present, the former is skipped, e.g.::
+   * When a class and its subclass are present, the latter is skipped, e.g.::
 
        Union[int, object] == object