Fix the implemented varaints section in PPO (#193)

Author: vwxyzjn

docs/rl-algorithms/ppo.md

@@ -29,6 +29,8 @@ All our PPO implementations below are augmented with the same code-level optimiz
 | :material-github: [`ppo_atari_lstm.py`](https://github.com/vwxyzjn/cleanrl/blob/master/cleanrl/ppo_atari_lstm.py), :material-file-document: [docs](/rl-algorithms/ppo/#ppo_atari_lstmpy) | For Atari games using LSTM without stacked frames. |
 | :material-github: [`ppo_atari_envpool.py`](https://github.com/vwxyzjn/cleanrl/blob/master/cleanrl/ppo_atari_envpool.py), :material-file-document: [docs](/rl-algorithms/ppo/#ppo_atari_envpoolpy) | Uses the blazing fast Envpool Atari vectorized environment. |
 | :material-github: [`ppo_procgen.py`](https://github.com/vwxyzjn/cleanrl/blob/master/cleanrl/ppo_procgen.py), :material-file-document: [docs](/rl-algorithms/ppo/#ppo_procgenpy) | For the procgen environments |
+| :material-github: [`ppo_atari_multigpu.py`](https://github.com/vwxyzjn/cleanrl/blob/master/cleanrl/ppo_atari_multigpu.py),  :material-file-document: [docs](/rl-algorithms/ppo/#ppo_atari_multigpupy)| For Atari environments leveraging multi-GPUs |
+| :material-github: [`ppo_pettingzoo_ma_atari.py`](https://github.com/vwxyzjn/cleanrl/blob/master/cleanrl/ppo_pettingzoo_ma_atari.py),  :material-file-document: [docs](/rl-algorithms/ppo/#ppo_pettingzoo_ma_ataripy)| For Pettingzoo's multi-agent Atari environments |
 
 Below are our single-file implementations of PPO: