docs: use WantedBy in kmsconvt@.service

We currently use the old systemd syntax that was copied from
getty@.service. It doesn't allow enabling specific TTYs, though. So use
the new WantedBy syntax instead.

Thanks to "trusktr" for investigating and reporting upstream to systemd
developers.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>