fixes OpenAPI spec mismatches between require properties and available properties

resolves #1222

Summary

Updates the OpenAPI spec files for the outputs used in DocC archive to resolve warnings in mismatches between required fields and available properties defined in those fields

Updates:

• LinkableEntities.json
• RenderNode.spec.json

Dependencies

none

Testing

Manually verified the output doesn't present warnings when code generation is run with OpenAPI generator:

An example command:

git clone https://github.com/apple/swift-openapi-generator
cd swift-openapi-generator
swift run swift-openapi-generator generate --mode types --output-directory ~/foo /Users/joeheck/src/swift-docc/Sources/SwiftDocC/SwiftDocC.docc/Resources/LinkableEntities.json

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary