I don't know if that's possible, but I really don't think it's a big deal. I find it hard to believe a medical school would actually believe you'd purposely make that sort of punctuation error. They'll probably overlook it. Besides, how much time are they really spending reading these essays? They may easily overlook the fact that it's a period instead of a comma. I wouldn't worry so much! 🙂 But if you do withdraw and resubmit (if that's even possible), good luck!