Because "dites" is the 2nd plural, it corresponds to the pronoun "vous". You don't change anything with this pronoun. Usually, you don't even think about it because words end with "-ez" (ex: mangez), but "dites" is an irregular conjugation. But STILL, it corresponds to the 2nd plural ("vous") so you can't change anything about the way you write it.
Hope it's clearer now !