Fix the names of some FreeBSD sysctls in include/tunables.cfg (#16395)

Sponsored-by: Klara, Inc.

Signed-off-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>