[RLlib] Fix doc test for foreach_worker_with_index

[ArturNiederfahrenhorst]

Signed-off-by: Artur Niederfahrenhorst artur@anyscale.com

WorkerSet no long has foreach_worker_with_index

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
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• Testing Strategy
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[maxpumperla]

@ArturNiederfahrenhorst cool, thanks for fixing this! btw, we have a lot of other dead code like this on our docs. this was just a fairly mindless copy (from rst to py) of what has been up for maybe years. We should really get after that soonish, for the sake of managing retention.