fix comments on the metrics in voxel.py
fix comments on the metrics in voxel.py

StongeEtienne/dipy,demianw/dipy,matthieudumont/dipy,Messaoud-Boudjada/dipy,matthieudumont/dipy,nilgoyyou/dipy,sinkpoint/dipy,beni55/dipy,demianw/dipy,FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy,jyeatman/dipy,beni55/dipy,mdesco/dipy,villalonreina/dipy,sinkpoint/dipy,Messaoud-Boudjada/dipy,jyeatman/dipy,mdesco/dipy,samuelstjean/dipy,rfdougherty/dipy,StongeEtienne/dipy,JohnGriffiths/dipy,oesteban/dipy,oesteban/dipy,JohnGriffiths/dipy,samuelstjean/dipy,rfdougherty/dipy,nilgoyyou/dipy,villalonreina/dipy,samuelstjean/dipy