sysadmin: fix the device declaration in the bonding example

sysadmin: fix the device declaration in the bonding example

We already have a "Use a bond as bridge port" example below this
one, the from reading this ones description it's cleanly meant to be
a extra bond, not on a bridge for VM internal traffic, but for things
like ceph or VM migration traffic.

Signed-off-by: Wolfgang Link <w.link@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>