tidb-configuration-file: add unit for log.max-size

[hey-hoho]

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What is changed, added or deleted? (Required)

Update tidb-configuration-file.md.
The description of log.max-size is not clear, and the default unit is added.

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

LGTM

[hey-hoho]

good job.

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: cfa237a