Fix wrong device path in NFSv4 volume example (#10429)

In section Create a service which creates an NFS volume,
NFSv3 command states right device path(:/var/docker-nfs),
but NFSv4 command is not configured to refer that path.
So updated the path.

storage/volumes.md

@@ -463,7 +463,7 @@ $ docker service create -d \
 ```bash
 docker service create -d \
     --name nfs-service \
-    --mount 'type=volume,source=nfsvolume,target=/app,volume-driver=local,volume-opt=type=nfs,volume-opt=device=:/,"volume-opt=o=10.0.0.10,rw,nfsvers=4,async"' \
+    --mount 'type=volume,source=nfsvolume,target=/app,volume-driver=local,volume-opt=type=nfs,volume-opt=device=:/var/docker-nfs,"volume-opt=o=10.0.0.10,rw,nfsvers=4,async"' \
     nginx:latest
 ```