BF: fix doctest for val_vec_sum
BF: fix doctest for val_vec_sum

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