Fix CodegenSchema Enum type to not export array (#47324)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47324

I need to reference this type somewhere else, but not an array of the type.

Generally we prefer that all exported types are the object itself, and it used as a member type of arrays when used.

Changelog: [Internal]

Reviewed By: makovkastar

Differential Revision: D65259014

fbshipit-source-id: 35fb5fe03a44bed61ad87337d0fc5c198744c0e9