DOC: updated slicing example
DOC: updated slicing example

JohnGriffiths/dipy,matthieudumont/dipy,matthieudumont/dipy,JohnGriffiths/dipy,FrancoisRheaultUS/dipy,villalonreina/dipy,StongeEtienne/dipy,StongeEtienne/dipy,FrancoisRheaultUS/dipy,villalonreina/dipy,nilgoyyou/dipy,nilgoyyou/dipy