Update docker swarm doc to use docker volume prune (#5053)
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.
abperiasamy/minio,vadmeste/minio,aead/minio,vadmeste/minio,fwessels/minio,minio/minio,rushenn/minio,jimjag/minio,krishnasrinivas/minio,harshavardhana/minio,rushenn/minio,balamurugana/minio,fwessels/minio,NitishT/minio,donatello/minio,fwessels/minio,harshavardhana/minio,aead/minio,NitishT/minio,aead/minio,harshavardhana/minio,harshavardhana/minio,rushenn/minio,aead/minio,jimjag/minio,minio/minio,aead/minio,jimjag/minio,harshavardhana/minio,rushenn/minio,fwessels/minio,krisis/minio,donatello/minio,NitishT/minio,aead/minio,balamurugana/minio,harshavardhana/minio,harshavardhana/minio,aead/minio,jimjag/minio,aead/minio,NitishT/minio,krishnasrinivas/minio,harshavardhana/minio,abperiasamy/minio,krishnasrinivas/minio,krisis/minio,abperiasamy/minio,abperiasamy/minio,krishnasrinivas/minio