Tweak GenAI attribution PR check wording

[jfrost-mo]

Minor wording tweak to GenAI checklist item.

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [x] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [ ] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Said whether Generative AI, such as GitHub Copilot, has been used in this PR.
• [x] Marked the PR as ready to review.

[jfrost-mo]

Merging without review, as this is a tiny documentation change that isn't part of the code itself.

[github-actions[bot]]