minor typo fix in layout docs
minor typo fix in layout docs

matham/kivy,matham/kivy,akshayaurora/kivy,akshayaurora/kivy,kivy/kivy,rnixx/kivy,kivy/kivy,matham/kivy,kivy/kivy,akshayaurora/kivy,rnixx/kivy,rnixx/kivy,matham/kivy