Container source page points to wrong path for ko publish command.

[ScrapCodes]

Instruction on the page: https://knative.dev/development/eventing/samples/container-source/, needs to be updated.

Steps to Reproduce the Problem

ko publish github.com/knative/eventing-contrib/cmd/heartbeats
2019/08/22 19:25:51 failed to publish images: importpath "github.com/knative/eventing-contrib/cmd/heartbeats" is not supported

Expected.

ko publish knative.dev/eventing-contrib/cmd/heartbeats
2019/08/22 19:26:08 Building knative.dev/eventing-contrib/cmd/heartbeats
2019/08/22 19:26:11 Using base gcr.io/distroless/static:latest for knative.dev/eventing-contrib/cmd/heartbeats
2019/08/22 ...

Install information:

• Knative Version: master