This updates example `kubectl exec` commands to avoid warnings.
It is a follow-up change to kubernetes#37221.
Running `kubectl exec <pod_name> <command>` is deprecated:
```
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead
```
It should be: `kubectl exec <pod_name> -- <command>`
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