Linguistic note: it's not misspelled at all. It's the historically correct spelling in both English and French, since it comes from Latin stilus. The 'y' spelling is more recent, and is a "misspelling" due to a folk etymology: the belief that it's somehow related to the Greek word στῦλος.

My bad. I had at first assumed that Voltaire's use of 'stile' was similar to his (apocryphal) use of 'poëshie' to make fun of Freytag's French, but checked by search and it's in lots of his letters. That's an interesting linguistic history!