As noted, 'A' class on Turkish is an Economy booking code. It's a somewhat rare display bug on the United website to display it as First, and this is because almost all *A carriers use 'A' class to denote Discount First (exceptions are those carriers with no F cabin, including TK, AC, and NZ).

If you thought you were purchasing a ticket in First, it's worth complaining about the misrepresentation, but the price you paid would have been for Economy.