Fixing atomic engine warning message

[shabroo]

@alexey-milovidov please stop torturing me and accept this commit. :))

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixed grammar from "a" to "the" in the warning message. There is only one Atomic engine, so it should be "to the new Atomic engine" instead of "to a new Atomic engine".

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

---

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[clickhouse-ci]

This is an automatic comment. The PR descriptions does not match the template.

Please, edit it accordingly.

The error is: Changelog entry required for category 'Improvement'

[robot-clickhouse-ci-1]

This is an automated comment for commit 8e8bc83 with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
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[alexey-milovidov]

Perfect.