Fix archive link

README.md

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 References
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-* Michael Blondin, Javier Esparza, Stefan Jaax, Philipp J. Meyer: “Towards Efficient Verification of Population Protocols”, 2017; [http://arxiv.org/abs/1703.04367 arXiv:1703.04367].
+* Michael Blondin, Javier Esparza, Stefan Jaax, Philipp J. Meyer: “Towards Efficient Verification of Population Protocols”, 2017; [arXiv:1703.04367](http://arxiv.org/abs/1703.04367).
 
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