doc: update node migration guide

last thing:
+$ export ETCDCTL_PEERS=http://10.0.1.10:2379,http://10.0.1.11:2379,http://10.0.1.11:2379 ->
+$ export ETCDCTL_PEERS=http://10.0.1.10:2379,http://10.0.1.11:2379,http://10.0.1.12:2379