ldd: clarify format options

-f may be specified zero, one, or two times. Make this clear in the
usage.

Reviewed by: markj, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34733

(cherry picked from commit e9f8723c30fc390f1282f5286d57c090ed9e5925)