docs(cloud_firestore): update changelog

[creativecreatorormaybenot]

Description

This fixes something that I introduced in #2931. It seems like there was a very weird Git diff bug there because I had it exactly like this in my local file 😶 (the diff on GitHub showed the opposite, i.e. [UNPUBLISHED] was still there and I added the version 🤷🏽‍♀️

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[creativecreatorormaybenot]

You can even still see the GitHub diff here. It seems like it ran on an outdated head.

[Salakar]

Thank you :)