sh: Fix pipebuf limit

Since the factor is not 1, we need to provide a unit.

MFC after: 1 week
Fixes: 5d92f20c7d31 ("bin/sh: support RLIMIT_PIPEBUF")
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D57352

(cherry picked from commit dfd2273d27627313f944650840381e878077e825)