doc: fix typo in window docs
doc: fix typo in window docs

matham/kivy,LogicalDash/kivy,matham/kivy,akshayaurora/kivy,kivy/kivy,akshayaurora/kivy,LogicalDash/kivy,inclement/kivy,inclement/kivy,rnixx/kivy,LogicalDash/kivy,matham/kivy,LogicalDash/kivy,akshayaurora/kivy,kivy/kivy,kivy/kivy,matham/kivy,rnixx/kivy,rnixx/kivy,inclement/kivy