doc/articles/image_draw.html: fix circle example

It was showing the same snippet twice instead of the type definition and snippet.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5875045

doc/progs/image_draw.go

@@ -117,7 +117,7 @@ func Glyph() {
 	// STOP OMIT
 }
 
-//CIRCLE OMIT
+//CIRCLESTRUCT OMIT
 type circle struct {
 	p image.Point
 	r int
@@ -139,4 +139,4 @@ func (c *circle) At(x, y int) color.Color {
 	return color.Alpha{0}
 }
 
-//STOP
+//STOP OMIT