Update migrate-large-mysql-to-tidb.md (#21135)

[ti-chi-bot]

This is an automated cherry-pick of #21135

refer to : https://docs.pingcap.com/zh/tidb/stable/get-started-with-tidb-lightning/#%E7%AC%AC-4-%E6%AD%A5%E5%90%AF%E5%8A%A8-tidb-lightning

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refer to :
https://docs.pingcap.com/zh/tidb/stable/get-started-with-tidb-lightning/#%E7%AC%AC-4-%E6%AD%A5%E5%90%AF%E5%8A%A8-tidb-lightning

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[hfxsd]

/approve

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