Fix broken links to `atc/worker.go`

[markdouglasswork]

The current 'payload' links brought me to a 404 page. I think I found the intended destination, but you may want to verify.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[jncd]

Thanks @markthemarkest