Update bw_monitor_13.py

Traceback (most recent call last):
  File "./bin/ryu-manager", line 19, in <module>
    main()
  File "/home/ubuntu/ryu/ryu/cmd/manager.py", line 75, in main
    services.extend(app_mgr.instantiate_apps(**contexts))
  File "/home/ubuntu/ryu/ryu/base/app_manager.py", line 434, in instantiate_apps
    self._instantiate(app_name, cls, *args, **kwargs)
  File "/home/ubuntu/ryu/ryu/base/app_manager.py", line 420, in _instantiate
    app = cls(*args, **kwargs)
  File "/home/ubuntu/ryu/ryu/app/bw_monitor_13.py", line 13, in __init__
    super(SimpleMonitor, self).__init__(*args, **kwargs)
NameError: global name 'SimpleMonitor' is not defined