update the variable scope (#15655)

[ti-chi-bot]

This is an automated cherry-pick of #15655

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• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.4 (TiDB 7.4 versions)
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What is the related PR or file link(s)?

• This PR is translated from: update the variable scope  docs-cn#15655
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ran-huang]

/approve

[ti-chi-bot]

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This pull-request has been approved by: ran-huang

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[ran-huang]

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