node-mixin: fix query in Disk Space Utilisation dashboard
node-mixin: fix query in Disk Space Utilisation dashboard

Signed-off-by: Sergiusz Urbaniak <7657f3a0c6319088c9e0bc4813152bb1f5637169@gmail.com>

siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter