Update citation sources in Readme.md (#5909)

Author: senden9

docs/Readme.md

@@ -82,6 +82,7 @@ cite the following paper as a reference:
   title={Unity: A general platform for intelligent agents},
   author={Juliani, Arthur and Berges, Vincent-Pierre and Teng, Ervin and Cohen, Andrew and Harper, Jonathan and Elion, Chris and Goy, Chris and Gao, Yuan and Henry, Hunter and Mattar, Marwan and Lange, Danny},
   journal={arXiv preprint arXiv:1809.02627},
+  url={https://arxiv.org/pdf/1809.02627.pdf},
   year={2020}
 }
 ```
@@ -91,9 +92,10 @@ cite the following paper as a reference:
 
 ```
 @article{cohen2022,
-  title={On the Use and Misuse of Abosrbing States in Multi-agent Reinforcement Learning},
+  title={On the Use and Misuse of Absorbing States in Multi-agent Reinforcement Learning},
   author={Cohen, Andrew and Teng, Ervin and Berges, Vincent-Pierre and Dong, Ruo-Ping and Henry, Hunter and Mattar, Marwan and Zook, Alexander and Ganguly, Sujoy},
   journal={RL in Games Workshop AAAI 2022},
+  url={http://aaai-rlg.mlanctot.info/papers/AAAI22-RLG_paper_32.pdf},
   year={2022}
 }
 ```