docs: use correct hints per bus type in <disk> examples

docs: use correct hints per bus type in <disk> examples

Commit 4cf53158 tried to set up unique labels per disk in the
example, but ended up choosing strings that don't correspond
to the usual choice of bus types.  Tweak the strings once again.

* docs/formatdomain.html.in: Use preferred names.

Signed-off-by: Eric Blake <eblake@redhat.com>