You're probably not the only person in the country to have done this. Inform your English teachers and examination officer at school, and see if they can approach AQA to query the problem. So long as your essay was well-written and responds with the correct answer to the questions you answered, I'm sure the examiners will be able to figure out which question number you intended to write.
Most people in my sixth form forgot to tick the box saying which question they'd answered in most of their written subject exams. You'd think that by studying A Levels we'd know better than that but clearly not...
Everyone does it every year, don't panic too much about it. If the results do come back unexpected, you can obtain the paper and request for your paper to be remarked.