Improve CronJob yaml in SinkBinding example (#3190)

* Improve CronJob yaml in SinkBinding example

* use correct apiVersion
* fix schedule so it runs every minute like the comment suggests
* use busybox as default example image, following suit from [Kubernetes CronJob docs](https://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/)

* fix indentation

* use appropriate heartbeat image

docs/eventing/sources/sinkbinding.md

@@ -87,13 +87,13 @@ Create a `CronJob` object:
 
 1. Copy the sample YAML into a `cronjob.yaml` file:
     ```yaml
-    apiVersion: batch/v1
+    apiVersion: batch/v1beta1
     kind: CronJob
     metadata:
       name: heartbeat-cron
     spec:
       # Run every minute
-      schedule: "* * * * *"
+      schedule: "*/1 * * * *"
       jobTemplate:
         metadata:
           labels:
@@ -104,9 +104,9 @@ Create a `CronJob` object:
               restartPolicy: Never
               containers:
                 - name: single-heartbeat
-                  image: <FILL IN YOUR IMAGE HERE>
+                  image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/heartbeats
                   args:
-                    - --period=1
+                  - --period=1
                   env:
                     - name: ONE_SHOT
                       value: "true"