Fix 2 broken links in the docs

[qlijin]

There is 2 link is broken(404) in the docs.

https://github.com/kubernetes/community/blob/master/communication.md#social-media https://github.com/kubernetes-sigs/etcdadm/etcd-manager

update to:

https://github.com/kubernetes/community/tree/master/communication#slack https://github.com/kubernetes-sigs/etcdadm/tree/master/etcd-manager

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[qlijin]

/reopen