sdl: fixed some typos in struct_test.go
sdl: fixed some typos in struct_test.go

Signed-off-by: Lilis Iskandar <3de94fd28d904a22098fd49f2d9af76f176c5fe9@gmail.com>

phicode/go-sdl2,veandco/go-sdl2,phicode/go-sdl2,veandco/go-sdl2,phicode/go-sdl2