doc: Deployment manifest page lists metallb config

```
deployment:
  addons:
    metallb: 192.168.123.81-192.168.123.90
```

I believe this should now be:

```
core:
  config:
    k8s-addons:
      loadbalancer: 192.168.123.81-192.168.123.90
```

---
Document: explanation/deployment-manifest.rst