fix(frontends/lean/pp): missing comma when printing contexts
fix(frontends/lean/pp): missing comma when printing contexts

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

rlewis1988/lean,digama0/lean,johoelzl/lean,sp3ctum/lean,leodemoura/lean,htzh/lean,soonhokong/travis_test,leanprover-community/lean,c-cube/lean,eigengrau/lean,c-cube/lean,leodemoura/lean,fpvandoorn/lean,johoelzl/lean,fpvandoorn/lean2,digama0/lean,UlrikBuchholtz/lean,Kha/lean,eigengrau/lean,c-cube/lean,dselsam/lean,fgdorais/lean,htzh/lean,leodemoura/lean,dselsam/lean,soonhokong/travis_test,leanprover/lean,digama0/lean,leanprover-community/lean,rlewis1988/lean,fpvandoorn/lean,codyroux/lean0.1,UlrikBuchholtz/lean,UlrikBuchholtz/lean,avigad/lean,soonhokong/lean-windows,soonhokong/lean-osx,leanprover-community/lean,sp3ctum/lean,leodemoura/lean,leodemoura/lean,htzh/lean,fgdorais/lean,fpvandoorn/lean,leanprover-community/lean,soonhokong/lean-windows,fgdorais/lean,rlewis1988/lean,digama0/lean,soonhokong/lean,soonhokong/lean-windows,javra/lean,sp3ctum/lean,soonhokong/lean-windows,soonhokong/lean-osx,avigad/lean,sp3ctum/lean,c-cube/lean,soonhokong/lean,levnach/lean,fgdorais/lean,leanprover-community/lean,codyroux/lean0.1,UlrikBuchholtz/lean,fpvandoorn/lean2,leanprover/lean,rlewis1988/lean,eigengrau/lean,soonhokong/lean,htzh/lean,dselsam/lean,fpvandoorn/lean2,javra/lean,levnach/lean,codyroux/lean0.1,levnach/lean,leanprover/lean,leanprover/lean,dselsam/lean,fpvandoorn/lean2,avigad/lean,dselsam/lean,soonhokong/lean-osx,fpvandoorn/lean2,johoelzl/lean,johoelzl/lean,leanprover-community/lean,UlrikBuchholtz/lean,sp3ctum/lean,soonhokong/travis_test,Kha/lean,fpvandoorn/lean,rlewis1988/lean,htzh/lean,javra/lean,johoelzl/lean,soonhokong/lean-osx,avigad/lean,levnach/lean,fgdorais/lean,fpvandoorn/lean,fgdorais/lean,eigengrau/lean,johoelzl/lean,soonhokong/lean-windows,rlewis1988/lean,soonhokong/lean,avigad/lean,dselsam/lean,javra/lean,soonhokong/travis_test,soonhokong/lean,leanprover/lean,Kha/lean,javra/lean,Kha/lean,c-cube/lean,digama0/lean,digama0/lean,Kha/lean,fpvandoorn/lean,soonhokong/lean-osx,leodemoura/lean,leanprover/lean,eigengrau/lean,avigad/lean,Kha/lean,levnach/lean,codyroux/lean0.1