doc: simplify the image_draw article example for converting an image to

RGBA.

R=adg, r, bsiegert
CC=golang-dev
https://golang.org/cl/6119054

doc/progs/image_draw.go

@@ -84,8 +84,8 @@ func ConvAndCircle() {
 
 	// CONV OMIT
 	b := src.Bounds()
-	m := image.NewRGBA(image.Rect(0, 0, b.Dx(), b.Dy()))
-	draw.Draw(m, m.Bounds(), src, b.Min, draw.Src)
+	m := image.NewRGBA(b)
+	draw.Draw(m, b, src, b.Min, draw.Src)
 	// STOP OMIT
 
 	p := image.Point{100, 100}