rotate an object with keyboard

I read this question How to rotate an object around a fixed point so it follows the mouse cursor? - Questions & Answers - Unity Discussions
and i got the idea but i want to do the same thing but with keyboard how can i do that i try a lot of things put not worked

public Vector3 point; //position of the point you want to rotate around

void Update () { 
       if (Input.GetKey(KeyCode.A)){
           transform.RotateAround(point, Vector3.up, 20 * Time.deltaTime);
       } else if (Input.GetKey(KeyCode.D))
           transform.RotateAround(point, -Vector3.up, 20 * Time.deltaTime);