beep(1): Use AFMT_FLOAT

AFMT_FLOAT is supported by sound(4) as of FILLME, so use it here, since
the whole program works with floats already.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47639

(cherry picked from commit 1166cfd96f3403edb082387fbc3a4d4aa249bce8)