I second that this will not be a problem at all. 1 letter will not be an issue at checkin. You cannot rectify the spelling error in the ticket now without cancelling and new ticket being issued; but you don't want to pay for that and it not neccessary. As long as you state the correct name in the APIS at check-in you should be fine. It has no bearing/importance that you can't add the reservation to your M&M profile. I have had this before even without spelling errors in my name.