The rental agreement should have the license plate number on it. As long as that matches, there isn't anything to worry about.

In fact, there isn't anything to worry about period, but if you are the worrying type, and you find that the license plate on the rental agreement does not actually match the car you were given, you can just drop Avis an email to let them know. This way there is a paper trail should there ever be a problem.

Either way, don't worry!

Safe travels!