:bug: README error update

[yannis-mlgrn]

i had a error, i couldn't turn with my joystick. set_steering() function return the good angle when i move the axis.
The error was these line in the documentation :

 V.add(ctr,
      inputs=['cam/image_array'],
      outputs=['user/angle', 'user/throttle', 'user/mode', 'recording'],
      threaded=True)

we need to replace 'user/angle' by 'user/steering'
Result :

            V.add(
                ctr,
                inputs=['cam/image_array'],
                outputs=['user/steering', 'user/throttle', 'user/mode', 'recording'],
                threaded=True
            )

I use the 5.1 version of donkeycar