fix(chapter6): change constructor name in example
fix(chapter6): change constructor name in example

although in the standard library "pair" stands for "prod.mk", this example is clearer if using "prod.mk" directly

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