Update env_checker.py

To now throw a warning if using the start value of discrete action space

Author: JoshuaBluem

stable_baselines3/common/env_checker.py

@@ -32,7 +32,7 @@ def _check_non_zero_start(space: spaces.Space, space_type: str = "observation",
     :param key: When the observation space comes from a Dict space, we pass the
         corresponding key to have more precise warning messages. Defaults to "".
     """
-    if isinstance(space, (spaces.Discrete, spaces.MultiDiscrete)) and not _starts_at_zero(space):
+    if isinstance(space, spaces.MultiDiscrete) and not _starts_at_zero(space):
         maybe_key = f"(key='{key}')" if key else ""
         warnings.warn(
             f"{type(space).__name__} {space_type} space {maybe_key} with a non-zero start (start={space.start}) "