DOCS - update the signature of NonLinearDirectionFinder to match docs
DOCS - update the signature of NonLinearDirectionFinder to match docs

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