`SDL.mouseMask.Right` and `SDL.mouseMask.Middle` are the same

[tDwtp]

luasdl2/src/mouse.c

Line 403 in 6df26c7

 { "Middle",			SDL_BUTTON_RMASK	}, 

to:

	{ "Middle", SDL_BUTTON_MMASK },

Upon dumping the sdl module table, I found that the values of SDL.mouseMask.Right and SDL.mouseMask.Middle are both the same (4 -> SDL_BUTTON_RMASK).

Error is in the Line mentioned above.
(I do not know how to do pull requests and such, sorry)

[tDwtp]

Fix was merged.