Fix typo in windowing chapter.
Fix typo in windowing chapter.

oktayacikalin/pyglet,theblacklion/pyglet,mammadori/pyglet,oktayacikalin/pyglet,mammadori/pyglet,mammadori/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,theblacklion/pyglet,theblacklion/pyglet,mammadori/pyglet,theblacklion/pyglet,oktayacikalin/pyglet