It's hardly libellous, rather the reverse ! No image rights are involved. The main complaint will be the public's when they pay on the false representation that you are playing; that's the organizers' problem. Get on to them straight away, have them remove your name, say it was an error, and tell them you are annoyed at being shown on the advertising as playing when you are not, since that is a false statement to the ticket buying public and one which is likely to lead to people accusing you of backing out; two embarrassments which you are sure they would wish to avoid.