[container source] Fix the docs to point to correct 'ko publish' command

[ScrapCodes]

Fixes #1725

Proposed Changes

-ko publish github.com/knative/eventing-contrib/cmd/heartbeats
+ko publish knative.dev/eventing-contrib/cmd/heartbeats

[knative-prow-robot]

Hi @ScrapCodes. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ScrapCodes]

/assign @carieshmarie

[RichieEscarez]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RichieEscarez, ScrapCodes

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