fix link to nicolas paper

photonics/index.html

@@ -112,7 +112,7 @@
         Scattering matrix of arbitrarily shaped objects: combining finite
       elements and vector partial waves</a>.  JOSA A 35.8 1401-1409, 2018.
     <li><a name="6"></a>N. Marsic, H. De Gersem, G. Demésy, A. Nicolet, C. Geuzaine.
-      <a href="https://arxiv.org/abs/1807.02355">
+      <a href="https://iopscience.iop.org/article/10.1088/1367-2630/aab6fd">
       Modal analysis of the ultrahigh finesse Haroche QED cavity</a>.
       New Journal of Physics 20.4, 043058, 2018.
     <li><a name="7"></a>F. Zolla, G. Renversez, A. Nicolet.  Foundations of