False positive for "data corpus"

## Description
For the term "data corpus" Harper reports a missing possessive s, even though the phrase is correct.

## Screenshots

Reproduce the report by checking how Harper handles the phrase "data corpus" and confirm the missing-possessive-s warning. Trace the grammar rule that produces this diagnostic, then verify that the correct phrase is no longer reported while comparable possessive cases still are.

Written by the indexing model from the issue text.