I just did some googling - it's one of those "there's 5 or 6 different valid spellings because the fusion regions of english and french were a nightmare of inconsistency". Chevaler and Chevalier are both valid spellings, as are Chevalour, Chivaler, and a few others.
Chevalier is probably the most recognizeable spelling and I am curious why they didn't go with it, but Chevaler at least on 2 minutes of google research seems like it's probably the most authentic one to the times?