docs: Update README.md to show the difference after firestore updated

[leewalter]

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  ** This is a suggestion to improve the new user understanding.

• Ensure the tests and linter pass - DONE

• Code coverage does not decrease (if any source code was changed) - DONE

• Appropriate docs were updated (if necessary) - DONE

Fixes #<PR # 15> 🦕

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

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