Fix errors detected by the test case

The service account token one must have `serviceAccountName` specified
in order to be successfully validated.
The PV duplicate example can only refer to an existing volume.

content/en/examples/pods/storage/pv-duplicate.yaml

@@ -8,13 +8,15 @@ spec:
     - name: test
       image: nginx
       volumeMounts:
-        - name: site-data
+        # a mount for site-data
+        - name: config
           mountPath: /usr/share/nginx/html
           subPath: html
+        # another mount for nginx config
         - name: config
           mountPath: /etc/nginx/nginx.conf
           subPath: nginx.conf
   volumes:
     - name: config
       persistentVolumeClaim:
-        claimName: test-nfs-claim
+        claimName: test-nfs-claim