[docs]: groups only works if given as list

# Documentation request

Using a list works, wheel group is populated:
```
users:
- name: myuser
groups:
- wheel
```

does not work, wheel group is empty:
```
users:
- name: myuser
groups: wheel
```

---
*Reported from: https://cloudinit.readthedocs.io/en/latest/reference/yaml_examples/user_groups.html*