Correct spelling core.py L90

Removed "either" -- typo or grammatical error

gym/core.py

@@ -87,7 +87,7 @@ def step(self, action: ActType) -> Tuple[ObsType, float, bool, bool, dict]:
         """Run one timestep of the environment's dynamics.
 
         When end of episode is reached, you are responsible for calling :meth:`reset` to reset this environment's state.
-        Accepts an action and returns either a tuple `(observation, reward, terminated, truncated, info)`.
+        Accepts an action and returns a tuple `(observation, reward, terminated, truncated, info)`.
 
         Args:
             action (ActType): an action provided by the agent