s/mixier/mixer/g

[j3tm0t0]

fix brew package name

$ brew search sdl
==> Formulae
cdogs-sdl         neopop-sdl        sdl2 ✔            sdl2_mixer ✔      sdl_gfx           sdl_net           sdl_ttf
gowsdl            ocamlsdl          sdl2_gfx          sdl2_net          sdl_image         sdl_rtf           sdlpop
msdl              sdl               sdl2_image        sdl2_ttf ✔        sdl_mixer         sdl_sound

[AZO234]

Thanks!