docs: update README to reflect CronJobSource renamed to PingSource (#6175)

Since [knative/eventing#2412](knative/eventing#2412) updated the resource name from CronJobSource to PingSource, the documentation is misleading.

Author: LYSingD

docs/eventing/event-registry/README.md

@@ -100,7 +100,7 @@ EventTypes are created automatically when an event source is instantiated.
 
 Knative supports automatic registration of EventTypes for the following event sources:
 
-- CronJobSource
+- PingSource
 - ApiServerSource
 - GithubSource
 - GcpPubSubSource