DOC: Small typo in SNR example.
DOC: Small typo in SNR example.

Thought I would shove it in here as well. Addresses https://github.com/nipy/dipy/issues/350

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