DOC: fix typo
DOC: fix typo

I guess I have the eye for these things or something.
nilgoyyou/dipy,matthieudumont/dipy,FrancoisRheaultUS/dipy,StongeEtienne/dipy,villalonreina/dipy,FrancoisRheaultUS/dipy,StongeEtienne/dipy,nilgoyyou/dipy,villalonreina/dipy,matthieudumont/dipy