README typo fix

Important typo fix in README example code.

README.rst

@@ -59,7 +59,7 @@ client:
 
     if __name__ == '__main__':
         # wrap Flask application with engineio's middleware
-        app = socketio.Middleware(eio, app)
+        app = socketio.Middleware(sio, app)
 
         # deploy as an eventlet WSGI server
         eventlet.wsgi.server(eventlet.listen(('', 8000)), app)
@@ -74,4 +74,4 @@ Resources
 .. _socket.io-client: https://github.com/Automattic/socket.io-client
 .. _Eventlet: http://eventlet.net/
 .. _Documentation: http://pythonhosted.org/python-socketio
-.. _PyPI: https://pypi.python.org/pypi/python-socketio
+.. _PyPI: https://pypi.python.org/pypi/python-socketio