[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 6580c19bbbe7bc9bc0884699afd69184f523b32e)
diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index 1780739..1e48fec 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -939,7 +939,7 @@
 
        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