STYLE: Update template dashboard driver script comment to suggest using "-j -lN"

As suggested in [1], using the -l flag allow to keep the machine responsive
while maximizing the use of the cores when possible.

[1] http://www.kitware.com/blog/home/post/434

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21692 3bd1e089-480b-0410-8dfb-8563597acbee