[box for prisms] fix edge to face mapping

A test setup with a prism grid revealed that the edge to face mapping in
boxfvelementgeometry contained wrong orders for two edges.

Reviewed by Bernd.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13851 2fb0f335-1f38-0410-981e-8018bf24f1b0