Add missing import statement for socketio in docs (#289)

* Add missing import statement for socketio

* Remove incorrect import statement

Author: faheel

docs/intro.rst

@@ -63,8 +63,8 @@ asynchronous server:
 
 .. code:: python
 
-    import engineio
     import eventlet
+    import socketio
 
     sio = socketio.Server()
     app = socketio.WSGIApp(sio, static_files={