Update comments to match code
s/get_vdev_spec/make_root_vdev
The former doesn't exist anymore.
Sponsored by: iXsystems, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Ryan Moeller <ryan@freqlabs.com>
Closes #8759