text: Title: Small documentation error in AECEnv's step method
  Body: Hi there,
  
  The documentation for the step method in the AECEnv base class reads as follows: 
  ```
  '''
  Receives a dictionary of actions keyed by the agent name.
  Returns the observation dictionary, reward dictionary, done dictionary, and info dictionary,
  where each dictionary is keyed by the agent.
  '''
  ```
  However, from looking at the `api_test.py` script in `pettingzoo/test/` I don't think is accurate, as you should just pass in the action for the current agent and nothing should be returned. Please correct me if I have misunderstood!
  