Examples: auth/backend.py is calling add_authenticator incorrectly

Looks like the API has changed and it needs to call add_authenticator(...) with a IAuthenticator instance and not (u"wampcra", authid="xxxx", secret="xxxx").

python  examples/twisted/wamp/auth/backend.py 
2019-11-08T21:10:47+0800 ApplicationSession could not be instantiated: add_authenticator() got an unexpected keyword argument 'authid'
2019-11-08T21:10:47+0800 Traceback (most recent call last):
  File "/home/Projects/venv11/lib64/python3.7/site-packages/autobahn/wamp/websocket.py", line 60, in onOpen
    self._session = self.factory._factory()
  File "/home/Projects/venv11/lib64/python3.7/site-packages/autobahn/twisted/wamp.py", line 244, in create
    session = make(cfg)
  File "examples/twisted/wamp/auth/backend.py", line 53, in make
    u"wampcra", authid=u'username', secret=u'p4ssw0rd'
TypeError: add_authenticator() got an unexpected keyword argument 'authid'

2019-11-08T21:10:47+0800 failing WebSocket connection (code=1011): "WAMP Internal Error (add_authenticator() got an unexpected keyword argument 'authid')"
2019-11-08T21:10:47+0800 Main loop terminated.

Start in examples/twisted/wamp/auth/backend.py at make, where add_authenticator is called, and compare that call with the current add_authenticator API and IAuthenticator usage. Update the example so it no longer passes authid and secret as keyword arguments, then run the example and confirm the reported TypeError is gone.