[8.12](backport #3260) Fix fleet-server.reference.yml indentation

[mergify]

This is an automatic backport of pull request #3260 done by Mergify.
Cherry-pick of f58c107 has failed:

On branch mergify/bp/8.12/pr-3260
Your branch is up to date with 'origin/8.12'.

You are currently cherry-picking commit f58c107.
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Unmerged paths:
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	both modified:   fleet-server.reference.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elastic-sonarqube]

Quality Gate passed

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @pierrehilbert? 🙏