fix: indentation in alert manager integration docs

[gabriel-suela]

What this PR does

Fix indentation in alertmanager docs.

The parts that were adjusted were not following the pattern of the other examples in the docs.

Which issue(s) this PR closes

None

Checklist

• [ ] Unit, integration, and e2e (if applicable) tests updated
• [ ] Documentation added (or pr:no public docs PR label added if not required)
• [ ] Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will show up in the autogenerated release notes.

[CLAassistant]

All committers have signed the CLA.

[gabriel-suela]

@iskhakov