[Heartbeat] Fix reference yaml format

[emilioalvap]

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.

[elasticmachine]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @emilioalvap? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Duration: 53 min 2 sec

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[devcorpio]

LGTM

@emilioalvap how did we catch this bug? is there any context available?

[emilioalvap]

@devcorpio mainly a discuss forum: https://discuss.elastic.co/t/heartbeat-reference-documentation-has-extra-syntax/355925