Issue #16174: Fix suggested usage of dummy_threading module.

Patch by Berker Peksag.

Doc/library/dummy_threading.rst

@@ -17,7 +17,7 @@ Suggested usage is::
    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