Updated examples for children

[tschaub]

Proposed Changes:

This suggests a few minor changes to the examples in the STAC API - Children readme.

1. Remove trailing commas from JSON examples
2. Remove extent from catalog (to make it clearer that it is just a catalog and not a collection)
3. Make one of the children a collection (since it already had the required fields)

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• A CHANGELOG entry is not required.