[8.13](backport #38584) [Heartbeat] Fix reference yaml format

[mergify]

Proposed commit message

Fix heartbeat.reference.yml incorrect formatting.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

---

This is an automatic backport of pull request #38584 done by [Mergify](https://mergify.com).

[botelastic]

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[elasticmachine]

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