Link to PingSource page is 404

## Expected Behavior

Link to page to not 404

## Actual Behavior

In https://knative.dev/docs/eventing/broker-trigger/
Link to PingSource goes to https://knative.dev/docs/eventing/samples/ping-source/index.html which is 404

the page is indeed at:

https://github.com/knative/docs/blob/master/docs/eventing/samples/ping-source/README.md

