Find a bug in the example of Fmb in mpi.

Now it works well the results are equal when comparing mpi & no mpi vers.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/scalfmm/scalfmm/trunk@81 2616d619-271b-44dc-8df4-d4a8f33a7222