fix grammar in Drawer doc
@@ -14,7 +14,7 @@ package pixel
// Picture.
//
// Whenever you change the Triangles, call Dirty to notify Drawer that Triangles changed. You don't
-// need to notify Drawer about a change of Picture.
+// need to notify Drawer about a change of the Picture.
type Drawer struct {
Triangles Triangles
Picture Picture
