[ci skip] Fix PullRequest link on Rails 5.1 Release Notes

[tzmfreedom]

Summary

The correct link for datetime_field changes is #25469.
But now, the link point to encode_special_chars. So I fixed the link.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

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[kamipo]

Thanks!
In the future, you can add [ci skip] to your commit message for documentation-only changes to skip the Travis build.