Fix a typo in bsdinstall

A variable was misspelled resulting in chmod executing on the installer instead of on the target chroot

PR: 191402
Submitted by: Martin Simmons <martin@lispworks.com>
Approved by: brueffer
MFC after: 3 days
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D3061