Fixed a typo in multipole bal calculation.
Fixed a typo in multipole bal calculation.

hussainmkj/sixtracklib,hussainmkj/sixtracklib,dpellegr/sixtracklib,dpellegr/sixtracklib,SixTrack/SixTrackLib,dpellegr/sixtracklib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib