Fix prometheus-mixin dashboards to use grafanaDashboards

Signed-off-by: NMatthias Loibl <mail@matthiasloibl.com>

documentation/prometheus-mixin/dashboards.jsonnet

-local dashboards = (import 'mixin.libsonnet').dashboards;
+local dashboards = (import 'mixin.libsonnet').grafanaDashboards;
 
 {
   [name]: dashboards[name]

documentation/prometheus-mixin/dashboards.libsonnet

 local g = import 'grafana-builder/grafana.libsonnet';
 
 {
-  dashboards+: {
+  grafanaDashboards+:: {
     'prometheus.json':
       g.dashboard('Prometheus')
       .addMultiTemplate('job', 'prometheus_build_info', 'job')