Yes, that's right. For example, imagine that there are 500 doors instead of just 3. When you pick one, you have a 1/500 chance of picking the one with the car. Then, the person opens up 498 doors that do not contain the car. By then, switching would mean that you have a 499/500 chance of winning, whereas not switching, you'd still only have a 1/500 chance of winning.