fix: indentation in alert manager integration docs (#4562)

# 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

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## 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.

---------

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

docs/sources/integrations/alertmanager/index.md

@@ -110,17 +110,17 @@ Add receiver configuration to `prometheus.yaml` with the **OnCall Heartbeat URL*
   route:
   ...
       routes:
-      - match:
+        - match:
           alertname: heartbeat
         receiver: 'grafana-oncall-heartbeat'
         group_wait: 0s
         group_interval: 1m
         repeat_interval: 50s
   receivers:
-  - name: 'grafana-oncall-heartbeat'
+    - name: 'grafana-oncall-heartbeat'
     webhook_configs:
-  - url: https://oncall-dev-us-central-0.grafana.net/oncall/integrations/v1/alertmanager/1234567890/heartbeat/
-    send_resolved: false
+      - url: https://oncall-dev-us-central-0.grafana.net/oncall/integrations/v1/alertmanager/1234567890/heartbeat/
+        send_resolved: false
 ```
 
 ## Note about legacy integration