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Fix minor err in kubeadm
**What this PR does / why we need it**:
fix minor text error in kubeadm.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
When a Kubernetes cluster is instantiated in IPv6-only mode
via 'kubeadm init ...', the liveness probes that are
configured by kubeadm for kube-dns currently use IP:port
strings of "::1:53" and "::1:10053". These IP:port strings should
instead include brackets around the ::1 IPv6 loopback address, e.g.
"[::1]:53" and "[::1]:10053".
This change adds the necessary brackets around the ::1 IPv6 loopback
address.
Without this change, the kube-dns sidecar container interprets the
bracket-less strings as IPv4 IP:port strings that have too many
colons, and the kube-dns pod is restarted about once every 2 minutes.
fixes#56543
/area ipv6
/sig network
