Image.Shape.polygon moduledoc is wrong

[RyoWakabayashi]

:height is the height

image/lib/image/shape.ex

Lines 64 to 68 in 41c75e1

	* `:width` is the width of the canvas onto which the
	polygon is drawn. The default is `500` pixels.
	* `:height` is the width of the canvas onto which the
	polygon is drawn. The default is `500` pixels.