[freeflow][docu] Change sign of source term in documentation.
I think that the source and the gravity should have the same sign in this description. See l.167 of freeflow/navierstokes/staggered/localresidual.hh.
I think that the source and the gravity should have the same sign in this description. See l.167 of freeflow/navierstokes/staggered/localresidual.hh.