Update README.md

Author: srmnitc

README.md

@@ -15,7 +15,7 @@ Python library and command line tool for calculation of free energies.
 If you find calphy useful, please consider citing:  
 Menon, Sarath, Yury Lysogorskiy, Jutta Rogal, and Ralf Drautz. 
 [“Automated Free Energy Calculation from Atomistic Simulations.”](https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.5.103801) 
-Physical Review Materials 5(10), 2021
+Physical Review Materials 5(10), 2021  
 DOI: 10.1103/PhysRevMaterials.5.103801  
 
 Download bibtex [here](https://journals.aps.org/prmaterials/export/10.1103/PhysRevMaterials.5.103801?type=bibtex&download=true)