release: fix VMSIZE following 1ca8842f3ad9

truncate(1) is not case-sensitive with regard to setting the size
of a file. makefs(8), however, does not honor upper-case values.
Update release-specific files and the release(7) manual page to
reflect this.

MFC with: 1ca8842f3ad9
Submitted by: ehem_freebsd_m5p.com (original)
Differential Review: https://reviews.freebsd.org/D28979
Sponsored by: Rubicon Communications, LLC ("Netgate")