ps.1: Use ISO/IEC byte units

The ps(1) utility uses 1024 byte units, not 1000 byte units.

git grep KiB | wc -l
535
git grep KBytes | wc -l
39

Fixes: 20bdda14fa5c (Consistent usage of Kbyte unit)
MFC after: 3 days
Reviewed by: jsm, imp, mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49522