syncer(dm): Fix async flush log message (#4500)

[ti-chi-bot]

This is an automated cherry-pick of #4500

What problem does this PR solve?

Issue Number: ref #4287

What is changed and how it works?

Fixed the wrong log message about checkpoint point

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

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[ti-chi-bot]

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[lance6716]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c5ffc0f

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