Relax...Don't just send an email to them. Call them! Although Early Decision is binding and you are forced to go to that school if accepted (unless you have medical or financial reasons) you could probably talk to them soon and they can correct it. However, talk to them TODAY or TOMORROW. Don't wait because you could really blow your chances if they feel you waited too long. Alert them of your mistake and remember to be honest.
Also, is Cal Poly your first choice? If it is then I don't see why the binding is such a big deal
Good Luck!