I, too, am going to suggest that you "overlook" this and let it be. The "honest mistake" excuse is pretty sound, since if you didn't notice the first time there's no reason why you would go back and check for it after AMCAS verification. Realistically speaking, what are the chances medical schools are going to painstakingly check each and every grade, and even if they did catch this it's highly unlikely they would do anything, especially since AMCAS did not spot it - and it's AMCAS' job to look for deliberate misreporting and honest mistakes alike.