fix(workers): fix get_worker_id signature (#58)

Author: justprosh

workers.aqua

@@ -12,7 +12,7 @@ service DealWorker("worker"):
 
  -- Returns worker peer id associated with `deal_id`.
  -- Returns nil if worker doesn't exist.
- get_peer_id_opt(deal_id: ?string) -> PeerId
+ get_worker_id(deal_id: ?string) -> ?PeerId
 
  -- Removes worker with all deployed spells and services.
  -- Throws an error if worker doesn't exist.