update another instance where Kwant paper appears

content/paper.txt

@@ -2,4 +2,4 @@ Kwant reference
 ---------------
 
 C. W. Groth, M. Wimmer, A. R. Akhmerov, X. Waintal, *Kwant: a software package
-for quantum transport*, `arXiv:1309.2926 <http://arxiv.org/abs/1309.2926>`_.
+for quantum transport*, `New J. Phys. **16** 063065 <http://iopscience.iop.org/1367-2630/16/6/063065>`_.