chore: update generate pdf scripts (#839)

[ti-chi-bot]

This is an automated cherry-pick of #839

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What is changed, added, or deleted? (Required)

Since the structure of TOC.md has been changed, the parse method for generating PDF is failed. This PR is to fix the parse method.

And this issue caused by #831

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

• master (the latest development version)
• v5.3 (TiDB DM 5.3 versions)
• v2.0 (TiDB DM 2.0 versions)
• v1.0 (TiDB DM 1.0 versions)

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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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Liuxiaozhen12

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[Liuxiaozhen12]

/merge

[ti-chi-bot]

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

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Commit hash: 2ff7be3