node param in 'tiup dm scale-in' is mandatory, please add it in command help info

[seiya-annie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   [root@localhost ~]# tiup dm scale-in --help
   Scale in a DM cluster

Usage:
tiup dm scale-in [flags]

2. What did you expect to see?

tiup dm scale-in -N [flags]

3. What did you see instead?

Usage:
tiup dm scale-in [flags]
all flags are descripted as optional

4. What version of TiUP are you using (tiup --version)?

[root@localhost ~]# tiup --version
v1.0.9 tiup
Go Version: go1.13
Git Branch: master
GitHash: 41fbacf
[root@localhost ~]#
[root@localhost ~]# tiup dm --version
Starting component dm: /root/.tiup/components/dm/v1.1.2/tiup-dm --version
tiup version v1.1.2 tiup
Go Version: go1.13
Git Branch: release-1.1
GitHash: 7cb85b9-dirty
[root@localhost ~]#