Fix link to paper

Author: Mykola Zekter

README.md

@@ -1,7 +1,7 @@
 # Multi-Agent Particle Environment
 
 A simple multi-agent particle world with a continuous observation and discrete action space, along with some basic simulated physics.
-Used in the paper [Multi-Agent Actor-Critic for Mixed Cooperative-Competitive Environments](arxiv.org/pdf/1706.02275.pdf).
+Used in the paper [Multi-Agent Actor-Critic for Mixed Cooperative-Competitive Environments](https://arxiv.org/pdf/1706.02275.pdf).
 
 ## Getting started: