fix(chapter4): close code block
fix(chapter4): close code block

soonhokong/lean-tutorial,fpvandoorn/tutorial,avigad/tutorial,fpvandoorn/tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,avigad/tutorial,soonhokong/lean-tutorial,soonhokong/lean-tutorial,cjmazey/fork_leanprover_tutorial,avigad/tutorial,leanprover/tutorial,Hodapp87/cs6460_project,leanprover/tutorial,leanprover/tutorial,Hodapp87/cs6460_project