AO3-5776 Remove mention of author from download afterword

[Bilka2]

Pull Request Checklist

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-5776

Purpose

Replace "author" with "creator" in the download afterword that encourages commenting.

Did not do the bonus because counting the work creators looked quite complicated, at least when looking at the logic that bylines do. But maybe I missed some easy way to get all of a work's creators/external creators/anon status.

Testing Instructions

See Jira.

References

#4607 for the change to downloads/show.html.erb

Credit

Bilka (he/him)

[sarken]

Thanks!