Copter: manual throttle scaling fix

Scaling of bottom half of throttle was changed from THR_MIN ~ 500.
Previously it was from 0 ~ 500.

ArduCopter/Attitude.pde

@@ -853,7 +853,7 @@ static int16_t get_pilot_desired_throttle(int16_t throttle_control)
     // check throttle is above, below or in the deadband
     if (throttle_control < THROTTLE_IN_MIDDLE) {
         // below the deadband
-        throttle_out = (float)throttle_control * (float)g.throttle_mid / 500.0f;
+        throttle_out = g.throttle_min + ((float)(throttle_control-g.throttle_min))*((float)(g.throttle_mid - g.throttle_min))/((float)(500-g.throttle_min));
     }else if(throttle_control > THROTTLE_IN_MIDDLE) {
         // above the deadband
         throttle_out = g.throttle_mid + ((float)(throttle_control-500))*(float)(1000-g.throttle_mid)/500.0f;