Fix ref guide for autoscaling metric trigger

Reported in the IRC channel who ask me to raise an issue,

The documentation for the autoscaling metric trigger have an error on the description (it's not possible to set up a basic metric trigger with the current documentation).

https://lucene.apache.org/solr/guide/8_1/solrcloud-autoscaling-triggers.html#metric-trigger

metric:group:prefix should be replaced by metrics:group:prefix

This correction is also required on the example:

metrics:solr.node:CONTAINER.fs.coreRoot.usableSpace

This is confirmed by the source code with explicit use of "metrics:"  (see for example: org.apache.solr.cloud.autoscaling.sim.SimNodeStateProvider or org.apache.solr.cloud.autoscaling.MetricTriggerIntegrationTest)