Rename the dashboard in the mixin to 'Prometheus Overview'. (#7489)

Due to https://github.com/grafana/grafana/issues/15642, this prevents users putting this dashboard in a Grafana folder called 'Prometheus'.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

documentation/prometheus-mixin/dashboards.libsonnet

@@ -10,7 +10,7 @@ local template = grafana.template;
 {
   grafanaDashboards+:: {
     'prometheus.json':
-      g.dashboard('Prometheus')
+      g.dashboard('Prometheus Overview')
       .addMultiTemplate('job', 'prometheus_build_info', 'job')
       .addMultiTemplate('instance', 'prometheus_build_info', 'instance')
       .addRow(