IIWY, I would just edit the question (using the
edit link beneath your post, and leave a comment (mentioning the update) for the person who answered (i.e. Tom), by addressing them, using their username and preceding it with the
@ symbol (or, easier still, just leave a comment under their answer).
Tom will then be able to update his answer, so that both question and answer remain in-sync.
There is an edit history anyway (again, accessible via the
edited link beneath the post), if that is your concern, so all changes can be tracked and viewed (and rolled back to, if need be) at a later date.
However, Tom appears to have picked up your error already, and left you a comment.