Fix typo in image chapter.
Fix typo in image chapter.

adamlwgriffiths/Pyglet,niklaskorz/pyglet,seeminglee/pyglet64,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,niklaskorz/pyglet,seeminglee/pyglet64,niklaskorz/pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet