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/articles/image_draw.html

@@ -178,7 +178,7 @@ To draw an image through a circular mask with center <code>p</code> and radius
 <code>r</code>:
 </p>
 
-{{code "/doc/progs/image_draw.go" `/CIRCLE/` `/STOP/`}}
+{{code "/doc/progs/image_draw.go" `/CIRCLESTRUCT/` `/STOP/`}}
 {{code "/doc/progs/image_draw.go" `/CIRCLE2/` `/STOP/`}}
   
 <p>

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