Fixed small typo in user guide
Fixed small typo in user guide

Juanlu001/poliastro,Juanlu001/poliastro,anhiga/poliastro,Juanlu001/poliastro,anhiga/poliastro,newlawrence/poliastro,poliastro/poliastro,anhiga/poliastro,newlawrence/poliastro,newlawrence/poliastro