csg_boltzmann: fixed a typo in 'help hist set periodic'
csg_boltzmann: fixed a typo in 'help hist set periodic'

votca/csg,MrTheodor/csg,MrTheodor/csg,junghans/csg,votca/csg,junghans/csg,votca/csg,junghans/csg,junghans/csg,MrTheodor/csg,MrTheodor/csg,junghans/csg,votca/csg,MrTheodor/csg,junghans/csg