Merge pull request #1358 from Axelrod-Python/marcharper-patch-1

Update citations.md

Author: meatballs

citations.md

@@ -2,8 +2,9 @@
 
 ### 2020
 
+* Krapohl, Sebastian, Václav Ocelík, and Dawid M. Walentek. "The instability of globalization: applying evolutionary game theory to global trade cooperation." [Public Choice](https://ideas.repec.org/a/kap/pubcho/vyid10.1007_s11127-020-00799-1.html) (2020): 1-21.
 * Glynatsi, Nikoleta E., and Vincent A. Knight. "A meta analysis of tournaments and an evaluation of performance in the Iterated Prisoner's Dilemma" arXiv preprint arXiv:2001.05911 (2019). [ArXiv](https://arxiv.org/abs/2001.05911)
-* Wang, Shiheng, and Fangzhen Lin. "Nice Invincible Strategy for the Average-Payoff IPD." [pdf](https://www.aaai.org/Papers/AAAI/2020GB/AAAI-WangS.1218.pdf)
+* Wang, Shiheng, and Fangzhen Lin. "Nice Invincible Strategy for the Average-Payoff IPD." [Proceedings of the AAAI Conference on Artificial Intelligence](https://aaai.org/ojs/index.php/AAAI/article/view/5604)
 
 ### 2019