Change mappings value from a dictionary to a list in the default template

[Uko]

Resolves #316

Proposed changes

The current default template states that the mappings value of the map directive expects a dictionary or a list of dictionaries, while in reality, it works only with a list. As adding secondary support for a single dictionary looks like overkill, I propose to change the template, so both the demo value and a comment reflect that mappings should be used with a list of dictionaries.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have added Molecule tests that prove my fix is effective or that my feature works
• I have checked that any relevant Molecule tests pass after adding my changes
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md)

[alessfg]

Thanks for the PR! Can you update the Changelog too? Since this is the first PR scheduled for the next release, you will need to include:

## 0.6.1 (Unreleased)

as your changelog header.

[Uko]

@alessfg I added a changelog entry as requested

[alessfg]

LGTM!