examples: update platform snippets and doc

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

examples/minikube.jsonnet

@@ -1,6 +1,5 @@
 local kp =
   (import 'kube-prometheus/main.libsonnet') +
-  (import 'kube-prometheus/platforms/kubeadm.libsonnet') +
   // Note that NodePort type services is likely not a good idea for your production use case, it is only used for demonstration purposes here.
   (import 'kube-prometheus/addons/node-ports.libsonnet') +
   {
@@ -19,6 +18,9 @@ local kp =
           },
         },
       },
+      kubePrometheus+: {
+        platform: 'kubeadm',
+      },
     },
 
     // For simplicity, each of the following values for 'externalUrl':