Fix sample Prometheus alert config

doc/operations/metrics/alerts.md

@@ -80,10 +80,12 @@ section of your Prometheus Alertmanager configuration:
 
 ```yaml
 receivers:
-  name: gitlab
+  - name: gitlab
     webhook_configs:
       - http_config:
-        bearer_token: 9e1cbfcd546896a9ea8be557caf13a76
+          authorization:
+            type: Bearer
+            credentials: 9e1cbfcd546896a9ea8be557caf13a76
         send_resolved: true
         url: http://192.168.178.31:3001/root/manual_prometheus/prometheus/alerts/notify.json
         # Rest of configuration omitted