update citation information

Author: jgreener64

README.md

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 This repository contains the learned potential, simulation scripts and training code for the paper:
 
-Greener JG and Jones DT, Differentiable molecular simulation can learn all the parameters in a coarse-grained force field for proteins, bioRxiv (2021) - [link](https://www.biorxiv.org/content/10.1101/2021.02.05.429941v1)
+Greener JG and Jones DT, Differentiable molecular simulation can learn all the parameters in a coarse-grained force field for proteins, PLoS ONE 16(9): e0256990 (2021) - [link](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0256990)
 
 It provides the `cgdms` Python package which can be used to simulate any protein and reproduce the results in the paper.