Fix Typos in Metaschema Model for OSCAL SSP

[ohsh6o]

Committer Notes

Fixes #799.

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[ohsh6o]

I read the CONTRIBUTING.md document and thought I was mistaken about the ./src directory, let me know and I can close/relocate accordingly. Obviously, this is a minor fix in terms of priority anyway.

[ohsh6o]

Per conversation with @brianrufgsa, he is working on the M4 branch and will fix this there, instead of me needing to rebase from that branch instead of master. Will close later today.

[brian-ruf]

This will be addressed as part of PR #758.
Change has been made, but not yet merged with the metaschema-m4-integration branch associated with that PR.