Fix a 13 years old bug in an example

Trivial bug-fix for master, suggested by Eric. The bug was introduced by
a refactoring of the example, in the following commit, 13 years ago:

  | commit 898679a9e6
  | Author: Lutz Kettner <kettner@mpi-sb.mpg.de>
  | Date:   Thu Jul 12 05:59:09 2001 +0000
  |
  |     - Adapted to new polyhedral surface design (release 3).
  |     - Ported from dd_geo/chull to convex_hull_3.
  |     - Changed to new concept naming convention, i.e., WidthTraits_3.
  |     - Changed traits class to new kernel design naming. However, the unchanged
  |       member functions make this traits not compliant with kernel traits.
  |     - Edited Width_3.tex quite a bit. Removed redundant descriptions of type
  |       requirements that are actually described in the WidthTraits_3.
  |
  | Notes:
  |     r7838 trunk

Polytope_distance_d/examples/Polytope_distance_d/width_simplex.cpp

@@ -40,6 +40,7 @@ int main() {
     std::cout << "Direction: " << simplex.get_build_direction() << std::endl;
 
     Plane_3  e1, e2;
+    simplex.get_width_planes (e1, e2);
     std::cout << "Planes: E1: " << e1 << ".  E2: " << e2 <<std::endl;
 
     std::cout << "Number of optimal solutions: "