Incorrect YAML in document example

The difference between:

```python
{'disk': {'default': [{'format': 'qcow2',
                       'model': 'virtio',
                       'size': 8192,
                       'system': None}]}}
```

and:

```yaml
{'disk': {'default': [{'system': {'format': 'qcow2',
                                  'model': 'virtio',
                                  'size': 8192}}]}}
```