fix: webhook config for heartbeat

Author: henrywhitaker3

argo/cluster/monitoring/kube-prometheus-stack/chart/values.yaml

@@ -24,7 +24,7 @@ alertmanager:
       - name: "null"
       - name: heartbeat
         webhook_configs:
-          url_file: /etc/alertmanager/secrets/alertmanager-secrets/heartbeat
+          - url_file: /etc/alertmanager/secrets/alertmanager-secrets/heartbeat
       - name: discord_warn
         slack_configs:
           - api_url_file: /etc/alertmanager/secrets/alertmanager-secrets/slack_warn