Fix a heading level error

[lilin90]

What is changed, added or deleted? (Required)

Fixes a heading level error in Clustered Indexes | TiDB Docs

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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.

• [x] master (the latest development version)
• [x] v8.5 (TiDB 8.5 versions)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 versions)
• [ ] v8.2 (TiDB 8.2 versions)
• [x] v8.1 (TiDB 8.1 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [x] v7.1 (TiDB 7.1 versions)
• [x] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 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

[lilin90]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lilin90

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/pingcap/docs/blob/master/OWNERS)~~ [lilin90] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #19441.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19442.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #19443.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #19444.