text: Title: [Proposal] docs: gym -> gymnasium
  Body: ### Proposal
  
  A clear and concise description of the proposal.
  
  ### Motivation
  
  Please outline the motivation for the proposal.
  Is your feature request related to a problem? e.g.,"I'm always frustrated when [...]".
  If this is related to another GitHub issue, please link here too.
  
  ### Pitch
  
  A clear and concise description of what you want to happen.
  
  ### Alternatives
  
  A clear and concise description of any alternative solutions or features you've considered, if any.
  
  ### Additional context
  
  Add any other context or screenshots about the feature request here.
  
  ### Checklist
  
  - [ ] I have checked that there is no similar [issue](https://github.com/Farama-Foundation/PettingZoo/issues) in the repo (**required**)
  