correct the modal_title tag

I look at the calendar.js and find that when you set modal_title function in option, only <h3> will be used to
show the title. So if you keep the tag <h4>, the modal_title function will never work.

Besides, instead of find the <h3> tag in modal, I think it's better to find the tag who's class is 'modal-title'. But I don't know whether the change of the behavior will break something, so I just change h4 to h3.