user guide: replaced occurrences of 'xtfs_mount' and 'xtfs_mkvol'
user guide: replaced occurrences of 'xtfs_mount' and 'xtfs_mkvol'

gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs,gurkerl83/millipede-xtreemfs