doc: fix singular in sources/guide/lang.rst
doc: fix singular in sources/guide/lang.rst

Cheaterman/kivy,jegger/kivy,kived/kivy,vitorio/kivy,el-ethan/kivy,manthansharma/kivy,LogicalDash/kivy,rnixx/kivy,jegger/kivy,cbenhagen/kivy,rafalo1333/kivy,el-ethan/kivy,tony/kivy,autosportlabs/kivy,darkopevec/kivy,darkopevec/kivy,autosportlabs/kivy,jkankiewicz/kivy,arlowhite/kivy,jegger/kivy,Cheaterman/kivy,matham/kivy,arlowhite/kivy,bionoid/kivy,cbenhagen/kivy,bionoid/kivy,KeyWeeUsr/kivy,jkankiewicz/kivy,KeyWeeUsr/kivy,kivy/kivy,tony/kivy,LogicalDash/kivy,akshayaurora/kivy,andnovar/kivy,rafalo1333/kivy,rafalo1333/kivy,akshayaurora/kivy,el-ethan/kivy,kivy/kivy,janssen/kivy,aron-bordin/kivy,jkankiewicz/kivy,cbenhagen/kivy,kived/kivy,aron-bordin/kivy,manthansharma/kivy,LogicalDash/kivy,vitorio/kivy,autosportlabs/kivy,darkopevec/kivy,matham/kivy,jegger/kivy,janssen/kivy,manthansharma/kivy,vitorio/kivy,darkopevec/kivy,janssen/kivy,KeyWeeUsr/kivy,inclement/kivy,CuriousLearner/kivy,Cheaterman/kivy,kived/kivy,rnixx/kivy,akshayaurora/kivy,CuriousLearner/kivy,matham/kivy,LogicalDash/kivy,rnixx/kivy,andnovar/kivy,andnovar/kivy,tony/kivy,matham/kivy,bionoid/kivy,janssen/kivy,KeyWeeUsr/kivy,manthansharma/kivy,bionoid/kivy,kivy/kivy,arlowhite/kivy,inclement/kivy,aron-bordin/kivy,CuriousLearner/kivy,inclement/kivy,jkankiewicz/kivy,aron-bordin/kivy,Cheaterman/kivy