[Docs] Correct the example in the chapter (4.10.1. Configuring liveness and readiness probes)

The examples of the readiness probe and liveness probe using artemis check command-line fails because the env var are not substitued (see chapter 4.10.1 : Configuring liveness and readiness probes -> https://access.redhat.com/documentation/en-us/red_hat_amq_broker/7.10/html/deploying_amq_broker_on_openshift/assembly-br-configuring-operator-based-deployments_broker-ocp#proc-br-configuring-liveness-probe_broker-ocp.)

In order to use the env var we should execute the artemis command inside bash, which is aware of it's environment, like following:

readinessProbe:
      exec:
        command:
        - bash
        - -c
        - |
          /home/jboss/amq-broker/bin/artemis check node --silent --acceptor internal --user $AMQ_USER --password $AMQ_PASSWORD