Output unit schema inconsistent with spec

[tombentley]

The 2020-12 spec says this about successful validations:

The JSON key for failed validations is "error"; for successful validations it is "annotation".

A later sentence draws an explicit contrast for the case of nested results :

Note the plural forms, as a keyword with nested results can also have a local error or annotation.

But the output schema only defines a property for the plural annotations and not the singular annotation.