add cap style to arc documentation

Author: seb3s

lib/scenic/primitive/arc.ex

@@ -39,6 +39,7 @@ defmodule Scenic.Primitive.Arc do
   * [`hidden`](Scenic.Primitive.Style.Hidden.html) - show or hide the primitive
   * [`scissor`](Scenic.Primitive.Style.Scissor.html) - "scissor rectangle" that drawing will be clipped to.
   * [`fill`](Scenic.Primitive.Style.Fill.html) - fill in the area of the primitive
+  * [`cap`](Scenic.Primitive.Style.Cap.html) - says how to draw the ends of the arc.
   * [`stroke`](Scenic.Primitive.Style.Stroke.html) - stroke the outline of the primitive. In this case, only the curvy part.
 
   ## Usage