[ci skip] PPO1 Link Update (#51)

docs/modules/ppo1.rst

@@ -20,7 +20,7 @@ For that, ppo uses clipping to avoid too large update.
 Notes
 -----
 
--  Original paper:  https://arxiv.org/abs/1502.05477
+-  Original paper:  https://arxiv.org/abs/1707.06347
 - Clear explanation of PPO on Arxiv Insights channel: https://www.youtube.com/watch?v=5P7I-xPq8u8
 -  OpenAI blog post: https://blog.openai.com/openai-baselines-ppo/
 - ``mpirun -np 8 python -m stable_baselines.ppo1.run_atari`` runs the algorithm for 40M frames = 10M timesteps on an Atari game. See help (``-h``) for more options.