Fixed user_guide.rst title underline too short
Fixed user_guide.rst title underline too short

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