fix(src/library/definitional/equations.cpp): fix typo in error message
fix(src/library/definitional/equations.cpp): fix typo in error message

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