Merge issue #16174: Fix suggested usage of dummy_threading module.

Patch by Berker Peksag.
diff --git a/Doc/library/dummy_threading.rst b/Doc/library/dummy_threading.rst
index b578324..30a3ebb 100644
--- a/Doc/library/dummy_threading.rst
+++ b/Doc/library/dummy_threading.rst
@@ -17,7 +17,7 @@
    try:
        import threading
    except ImportError:
-       import dummy_threading
+       import dummy_threading as threading
 
 Be careful to not use this module where deadlock might occur from a thread being
 created that blocks waiting for another thread to be created.  This often occurs