Update 04_nginx-pvc.yaml

[jcwomack]

The current version of 04_nginx-pvc.yaml results in the pod being unable to run because the names PVC cannot be found, e.g.

$ kubectl describe pods/nginx-pvc
...
Events:
  Type     Reason            Age    From               Message
  ----     ------            ----   ----               -------
  Warning  FailedScheduling  3m37s  default-scheduler  0/6 nodes are available: persistentvolumeclaim "nginx-pvc-pod" not found. preemption: 0/6 nodes are available: 6 Preemption is not helpful for scheduling..
  Warning  FailedScheduling  3m36s  default-scheduler  0/6 nodes are available: persistentvolumeclaim "nginx-pvc-pod" not found. preemption: 0/6 nodes are available: 6 Preemption is not helpful for scheduling..

I think this is because he name given to the PVC is nginx-pvc, but the pod manifest specifies spec.volumes[*].persistentVolumeClaim: nginx-pvc-pod.

I've updated the names given to the PVC, pod, and volume to make them distinct and updated the persistentVolumeClaim value to the name of the PVC. This seems to fix the issue.