fix(frontends/lean/pp): use group for lambda/pi
fix(frontends/lean/pp): use group for lambda/pi

dselsam/lean,fgdorais/lean,digama0/lean,leodemoura/lean,johoelzl/lean,fgdorais/lean,fpvandoorn/lean,avigad/lean,leanprover/lean,fpvandoorn/lean,Kha/lean,soonhokong/lean-windows,fgdorais/lean,leanprover-community/lean,rlewis1988/lean,soonhokong/lean-osx,fpvandoorn/lean,soonhokong/lean-osx,dselsam/lean,digama0/lean,avigad/lean,soonhokong/lean-osx,leanprover/lean,leanprover/lean,digama0/lean,soonhokong/lean-osx,leanprover/lean,avigad/lean,leodemoura/lean,leodemoura/lean,avigad/lean,digama0/lean,soonhokong/lean-windows,dselsam/lean,rlewis1988/lean,rlewis1988/lean,johoelzl/lean,leodemoura/lean,fpvandoorn/lean,leanprover-community/lean,avigad/lean,leanprover-community/lean,Kha/lean,soonhokong/lean-windows,leanprover-community/lean,dselsam/lean,soonhokong/lean-windows,leodemoura/lean,fpvandoorn/lean,fgdorais/lean,dselsam/lean,Kha/lean,johoelzl/lean,leodemoura/lean,leanprover-community/lean,leanprover-community/lean,Kha/lean,soonhokong/lean-windows,soonhokong/lean-osx,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean,johoelzl/lean,fgdorais/lean,digama0/lean,Kha/lean,leanprover/lean,digama0/lean,leanprover/lean,Kha/lean,rlewis1988/lean,fgdorais/lean,dselsam/lean,avigad/lean,johoelzl/lean,rlewis1988/lean