examples: Fix additional rules snippets

examples/prometheus-additional-alert-rule-example.jsonnet

@@ -29,4 +29,5 @@ local kp = (import 'kube-prometheus/kube-prometheus.libsonnet') + {
 { ['kube-state-metrics-' + name]: kp.kubeStateMetrics[name] for name in std.objectFields(kp.kubeStateMetrics) } +
 { ['alertmanager-' + name]: kp.alertmanager[name] for name in std.objectFields(kp.alertmanager) } +
 { ['prometheus-' + name]: kp.prometheus[name] for name in std.objectFields(kp.prometheus) } +
+{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) } +
 { ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) }