Documentation: Review contents and English expression for doc/2_vehicle_model

[mohitk3000]

I have improved the english wordings, typos and explanation.
Also I found that:
in class State the method update() the paramter yaw_rate_rps comment was wrong.

Earlier

    def update(self, accel_mps2, yaw_rate_rps, time_s):
        """
        Function to update state
        accel_mps2: Acceleration[m/s^2]
        steer_rad: Steering angle[rad]
        time_s: Time interval per cycle[sec]
        """

Now

def update(self, accel_mps2, yaw_rate_rps, time_s):
        """
        Function to update state
        accel_mps2: Acceleration[m/s^2]
        yaw_rate_rps: Yaw rate[rad/s]
        time_s: Time interval per cycle[sec]
        """

I have updated at both doc file and State.py, verify this and give your feedback.
Thanks