Potential fix to Python code

When I try to run this in VS Studio python, I get an error when using "nonlocal" and it suggests using "global" here.

Author: heikora

articles/ai-services/speech-service/includes/how-to/recognize-speech/python.md

@@ -118,7 +118,7 @@ Now, create a callback to stop continuous recognition when `evt` is received. Ke
 def stop_cb(evt):
     print('CLOSING on {}'.format(evt))
     speech_recognizer.stop_continuous_recognition()
-    nonlocal done
+    global done
     done = True
 ```