quickstart: make DPDK devbind script usage consistent

Make the description of the DPDK devbind script in stop section of the
l3fwd application consistent with bind section.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>

Change-Id: I622ec304a19c26f3fc8a096ea338ac7e889ac43e