Update docker swarm doc to use docker volume prune (#5053)

docker prune command is an improvement over previous two step process
of removal of unused volumes in Swarm.

docs/orchestration/docker-swarm/README.md

@@ -51,16 +51,13 @@ Remove the distributed Minio services and related network by
 ```shell
 docker stack rm minio_stack
 ```
-Swarm doesn't automatically remove host volumes created for services. This may lead to corruption when a new Minio service is created in the swarm. So, we recommend removing all the volumes used by Minio, manually. To do this, go to each node in the swarm and list the volumes by 
+Swarm doesn't automatically remove host volumes created for services. This may lead to corruption when a new Minio service is created in the swarm. So, we recommend removing all the volumes used by Minio, manually. To do this, logon to each node in the swarm and run
 
 ```shell
-docker volume ls
+docker volume prune
 ```
-Then remove `minio_stack` volumes by 
+This will remove all the volumes not associated with any container.
 
-```shell
-docker volume rm volume_name 
-```
 
 ### Notes