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

usr.sbin/bsdinstall/scripts/zfsboot

@@ -1241,7 +1241,7 @@ zfs_create_boot()
 	local dir
 	for dir in /tmp /var/tmp; do
 		f_eval_catch $funcname chmod "$CHMOD_MODE" 1777 \
-		             $BSDINSTALL_CHROOTDIR$dir || return $FAILURE
+		             $BSDINSTALL_CHROOT$dir || return $FAILURE
 	done
 
 	# Create symlink(s)