libxc: fix meta

pkgs/development/libraries/libxc/default.nix

···
     homepage = http://octopus-code.org/wiki/Libxc;
     license = licenses.lgpl3;
     platforms = platforms.linux;
-    maintainer = with maintainers; [ markuskowa ];
+    maintainers = with maintainers; [ markuskowa ];
   };
 }