Schema generation touchups

[wendellpiez]

Committer Notes

Addresses #121. Also, the bug reported in usnistgov/OSCAL#844 disappears in this branch (possibly superseded since the Issue was written, but the generated schemas are still wrong in master).

This commit is a bug fix, but any other small XSD- or JSON-Schema-related issues could potentially be added.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Do all automated CI/CD checks pass?

Changes to Core Features:

n/a (no changes to core features)

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?
• Have you updated all website](https://pages.nist.gov/metaschema) and readme documentation affected by the changes you made? Changes to the website can be made in the website/content directory of your branch.

[david-waltermire]

LGTM.