vtfontcvt: correct usage

Commit e7fd9688ea97 changed vtfontcvt's command line parsing, but did
not correctly update the usage (it omitted the new -o flag).

Fixes: e7fd9688ea97 ("Move font related data structured to...")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit df777aeb9813cc37883fb6d4e1a4d502254b3193)