fix(unixode.sty): use \Sigma(∑) and \Pi(Π)
fix(unixode.sty): use \Sigma(∑) and \Pi(Π)

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