[DOCS] Update refs from master to main in USAGE.md etc

[djptek]

• Have you signed the contributor license agreement? Y
• Have you followed the contributor guidelines? Y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? Y
• If submitting code/script changes, have you verified all tests pass locally using make test? n/a
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? n(a
• Is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
• Have you added an entry to the CHANGELOG.next.md? Y

see: #1643

[ebeahan]

LGTM - thanks for catching!