chore(library/standard/logic/class/decidable): add missing 'end'
chore(library/standard/logic/class/decidable): add missing 'end'

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

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