should specify the namespace for pvc

We should specify the namespace for pvc, or users who use kube-ns will get in trouble.
(The pod has specified the namespace.)

examples/pvc.yaml

@@ -2,6 +2,7 @@ apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
   name: local-path-pvc
+  namespace: default
 spec:
   accessModes:
     - ReadWriteOnce