fix(css/tutorial.css): use freemono-lean font for <pre>
fix(css/tutorial.css): use freemono-lean font for <pre>

fix #60

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