Pin RabbitMQ image in JSON example to v3.9.4

since Concourse job test-examples is failing since it didn't pull a new
3.9-management image.

We need RabbitMQ >= 3.9.3 for this example to work.

Alternative (but less preferred option) is to set imagePullPolicy to Always.

Author: ansd

docs/examples/json-log/rabbitmq.yaml

@@ -4,7 +4,7 @@ metadata:
   name: json
 spec:
   # JSON logging is supported from RabbitMQ 3.9 onwards
-  image: rabbitmq:3.9-management
+  image: rabbitmq:3.9.4-management
   rabbitmq:
     # Disable RABBITMQ_LOGS=- which is set in
     # https://github.com/docker-library/rabbitmq/blob/ece63d4534cc44abd6b1ec35bbd9aa0d21e87e1d/3.9/ubuntu/Dockerfile#L211