Update migrate-large-mysql-to-tidb.md

[pepezzzz]

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

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

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

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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• 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

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