Correct component label of RabbitMQ PodMonitor

The bug got introduced in #671

Author: ansd

observability/prometheus/monitors/README.md

@@ -42,7 +42,7 @@ spec:
     interval: 15s
   selector:
     matchLabels:
-      app.kubernetes.io/component: rabbitmq-operator
+      app.kubernetes.io/component: rabbitmq
   namespaceSelector:
     any: true
 ```