Fix the command line of PBclust.py (issue #57)
Fix the command line of PBclust.py (issue #57)

Since the patch #45, the option for the number of clusters (-c) is now
mandatory in PBclust.py but the run_demo2.sh was not updated.

pierrepo/PBxplore,pierrepo/PBxplore,HubLot/PBxplore,HubLot/PBxplore,jbarnoud/PBxplore,jbarnoud/PBxplore