There should not be a semicolon ( ; ) at the end of this line:
public class MouseLook : MonoBehaviour;
Please try to pay closer attention to the code you are copying.
Similarly you have not copied the lines here properly:
float mouseX = Input.GetAxis("Mouse X") = mouseSensitivity = Time.deltaTime;
float mouseY = Input.GetAxis("Mouse Y") = mouseSensitivity = Time.deltaTime;
Also please use code tags in the future when posting code on the forums: Using code tags properly