AO3-6863 Update co-creator subject lines and tests

[boyer-victor]

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/AO3-6863

Purpose

Modifies the english locale and matching test to update the co-creator email subject lines.

Testing Instructions

How can the Archive's QA team verify that this is working as you intended?
Run the tests in spec/mailers/user_mailer_spec.rb

References

Are there other relevant issues/pull requests/mailing list discussions?

Credit

Jira Account: unsafe_deref, he/him

[brianjaustin]

Hi, unsafe_deref, thanks for the pull request!

The changes look good to me, so I've approved your PR. When we schedule this for release, it will be merged and sent to our manual QA team.

I've assigned the Jira issue to you and set its status to In Review so no one will mistakenly create a duplicate pull request. I've also given your account permission to comment on, assign, and transition issues in the future.

Thanks again for contributing! If you have any questions, you can contact us at [email protected].