[8.18] Fixed header level for DaC docs (backport #6694)

[mergify]

In the 8.18 docs, the header level was incorrectly set to an L3 when it should have been an L2. The header level is correct in the 9.0/Serverless docs.

Preview: https://security-docs_bk_6694.docs-preview.app.elstc.co/guide/en/security/8.x/detection-engine-overview.html#manage-rule-dac

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This is an automatic backport of pull request #6694 done by Mergify.

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