Both would be correct, so you can use either one if you'd like. It's just that "spelt" is used more in the UK, much like "burnt" or "learnt"... "Spelled" is used more in places such as the US or so.
(When I am speaking, I say "spelt", since I was born in the UK. Mostly when I am typing or writing, I use "spelled", since I was raised elsewhere. However, I think "spelled" sounds much better.)
