fix: update prompt_id to newspack_popup_id in the data events readme

[laurelfulford]

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Changes proposed in this Pull Request:

Updates the Data Events Readme as part of Automattic/newspack-popups#1509

This should be tested as part of Automattic/newspack-popups#1509 only because it shouldn't be merged until all of those changes are deemed okay.

Closes # .

How to test the changes in this Pull Request:

Other information:

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• Have you written new tests for your changes, as applicable?
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[github-actions]

Hey @laurelfulford, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

Please check if this PR needs to be included in the "Upcoming Changes" and "Release Notes" doc. If it doesn't, simply remove the label.

If it does, please add an entry to our shared document, with screenshots and testing instructions if applicable, then remove the label.

Thank you! ❤️