Fix run_id typing in trainer.py (#2537)

ml-agents/mlagents/trainers/trainer.py

         brain: BrainParameters,
         trainer_parameters: dict,
         training: bool,
-        run_id: int,
+        run_id: str,
         reward_buff_cap: int = 1,
     ):
         """
         :bool training: Whether the trainer is set for training.
-        :int run_id: The identifier of the current run
+        :str run_id: The identifier of the current run
         :int reward_buff_cap:
         """
         self.param_keys: List[str] = []