additional example for non default persistence service

For me it was confusing how to pass on the serviceId into methods that already had an argument. An extra example is always good.

configuration/persistence.md

@@ -193,6 +193,8 @@ You may specify a different persistence service by appending a String as an opti
 **Example**
 To persist an Item called `Lights` in an rrd4j database, you would enter the following:
 `Lights.persist("rrd4j")`
+To get the average temperature over the last 5 minues from the Item called `Temperature` in the influxdb persistence service, you would use:
+`Temperature.averageSince(now.minusMinutes(5,"influxdb"))`
 
 The most useful methods of the HistoricItem object returned by some queries, are `.state` and `.getTimestamp`