It's fine. Don't worry about it. You can write either 'Yogurt' or 'Yoghurt'. It really doesn't matter. This is a fairly new loan word which wasn't originally an English word, so there are different ways of spelling it. There are even different ways of saying it, too. Also, you needn't worry too much about minor spelling errors in the IELTS exam. Providing there aren't too many and they don't impede comprehension, you're unlikely to lose marks. I'm an IELTS examiner, by the way.

Yogurt is the preferred spelling in North America and Canada. Yoghurt is popular outside these countries.