Update pull secret steps in README

[Dan Clark]

Workshop suggests creating a new pull secret and using the following to inject it into the cluster

  SECRET=<the value you copied in step 4>

  oc apply -f - << EOF
    kind: Secret
    apiVersion: v1
    metadata:
      name: pull-secret
      namespace: openshift-config
    data:
      .dockerconfigjson: $SECRET
    type: kubernetes.io/dockerconfigjson
  EOF

This would likely fail since the pull-secret in openshift-config already exists. Probably want to provide a standin name.