docs/node-mixin/alerts: use a rate for network alerts
docs/node-mixin/alerts: use a rate for network alerts

Signed-off-by: paulfantom <5564a40b3044389cdd055bde6a926496e668fbf8@krupa.net.pl>

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