net: Fix documentation for unregister_netdevice_notifier_net

unregister_netdevice_notifier_net() is used for unregister a notifier
registered by register_netdevice_notifier_net(). Also s/into/from/.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>