ampoule.pool.ProcessPool misdocuments its "starter" attribute

`starter` is documented as "A process starter that implements L{iampoule.IStarter}.". However, what's actually expected is an object which "provides" `IStarter`, not "implements". In other words, the docs are asking for a class like `ampoule.main.ProcessStarter`, but they mean to be asking for an *instance* of such a class.