Fixes an obvious typo that passes 'f' instead of the user's firstname to the callback.
Hi!
That's indeed a very definite typo.
The interesting thing is we have deployed this and used this once. So it does *run*. But we've only used it to block creation of users, so the fact that it got the wrong data didn't matter :) (that is, every user would've gotten the first name 'f').
