[docs/update-grafana-dashboard-docs] Update Grafana dashboard instructions

Instructions to add Grafana dashboard do not work. The proposed
functions are wrong, according to
[grafana.libsonnet](https://github.com/brancz/kubernetes-grafana/blob/master/grafana/grafana.libsonnet)
`dashboards` and `rawDashboards` should be used in `grafana+::`
field.

This PR updates the existing documentation and fixes minor typos.

examples/grafana-additional-rendered-dashboard-example-2.jsonnet

@@ -2,8 +2,10 @@ local kp = (import 'kube-prometheus/kube-prometheus.libsonnet') + {
   _config+:: {
     namespace: 'monitoring',
   },
-  rawGrafanaDashboards+:: {
-    'my-dashboard.json': (importstr 'example-grafana-dashboard.json'),
+  grafana+:: {
+    rawDashboards+:: {
+      'my-dashboard.json': (importstr 'example-grafana-dashboard.json'),
+    },
   },
 };