doc/articles/image_draw.html: Change ColorImage to Uniform

Fixes #3474.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6048050

doc/articles/image_draw.html

@@ -89,7 +89,7 @@ interface which has a <code>Set</code> method.
 
 <p>
 To fill a rectangle with a solid color, use an <code>image.Uniform</code>
-source. The <code>ColorImage</code> type re-interprets a <code>Color</code> as a
+source. The <code>Uniform</code> type re-interprets a <code>Color</code> as a
 practically infinite-sized <code>Image</code> of that color. For those
 familiar with the design of Plan 9's draw library, there is no need
 for an explicit "repeat bit" in Go's slice-based image types; the
@@ -189,7 +189,7 @@ To draw an image through a circular mask with center <code>p</code> and radius
 
 <p> 
 To draw a font glyph in blue starting from a point <code>p</code>, draw with
-an <code>image.ColorImage</code> source and an <code>image.Alpha mask</code>. For
+an <code>image.Uniform</code> source and an <code>image.Alpha mask</code>. For
 simplicity, we aren't performing any sub-pixel positioning or
 rendering, or correcting for a font's height above a baseline.
 </p>