[docs]: groups only works if given as list

[jepper]

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

[holmanb]

It looks like we have tests that verify this behavior.

Please include logs and any other files required to demonstrate the issue that you are seeing.