DOC: Small typo in simulation doc
DOC: Small typo in simulation doc

Addresses https://github.com/nipy/dipy/issues/383

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