docs: Update containerd configuration format
`containerd` has adopted a new configuration style. Update the example configuration to reflect the change.
Fixes:
```toml
[plugins]
- [plugins.cri]
- [plugins.cri.containerd]
+ [plugins."io.containerd.grpc.v1.cri"]
+ [plugins."io.containerd.grpc.v1.cri".containerd]
default_runtime_name = "kata"
-
- [plugins.cri.containerd.runtimes.kata]
+ [plugins."io.containerd.grpc.v1.cri".containerd.runtimes]
+ [plugins."io.containerd.grpc.v1.cri".containerd.runtimes.kata]
runtime_type = "io.containerd.kata.v2"
```
