partedit: make minimum FreeBSD install size just under 1 GiB

1 GiB is a convenient disk image size for testing. It is also the
installer's minimum size, but the minimum applies to the partition
rather than the whole disk. Testing with a 1 GiB image resulted in the
counterintuitive error "There is not enough free space on <disk> to
install FreeBSD (1.0 GB free, 1.0 GB required)."

Reduce the installer's minimum size slightly to support this case.

Reviewed by: brd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38881