Fix item_asset and bands schema, fix typos

[m-mohr]

Related Issue(s): None

Proposed Changes:

1. item_assets were not properly validated, meaning most item_assets would fail without proper reason
2. Editorial changes (typos, etc)

Directly merging into master for a 1.1.0-beta.1 re-release as we haven't announced it yet...

PR Checklist:

• [ ] This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• [x] This PR has no breaking changes.
• [ ] I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• [ ] This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.

[m-mohr]

Found one additional issue with bands, don't merge yet.