worker.heartbeat: Update 'interval' doc default to 2 seconds

Since the 30-second docs were written in 3eaffda (Hartbeat is now 30
seconds, but documented as 2 minutes, 2011-06-13), the default has
changed twice:

* It was reduced to 5 seconds by 387cf1c (Heartbeat frequency now
  every 5s, and frequency sent with event, 2012-03-28).
* It was reduced to 2 seconds by 6008303 (Persist clock value and
  synchronize revoked tasks with others, 2012-11-05).
Merge pull request #1879 from wking/worker-heartbeat-docs

worker.heartbeat: Update 'interval' doc default to 2 seconds