image: fix obsolete reference in PalettedImage doc comment

PalettedColorModel was renamed to color.Palette
over three years ago by https://golang.org/cl/5132048.

Change-Id: I0204ade10eabff45620fda2990fed428c65d871e
Reviewed-on: https://go-review.googlesource.com/3305Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/image.go

@@ -46,9 +46,9 @@ type Image interface {
 }
 
 // PalettedImage is an image whose colors may come from a limited palette.
-// If m is a PalettedImage and m.ColorModel() returns a PalettedColorModel p,
+// If m is a PalettedImage and m.ColorModel() returns a color.Palette p,
 // then m.At(x, y) should be equivalent to p[m.ColorIndexAt(x, y)]. If m's
-// color model is not a PalettedColorModel, then ColorIndexAt's behavior is
+// color model is not a color.Palette, then ColorIndexAt's behavior is
 // undefined.
 type PalettedImage interface {
 	// ColorIndexAt returns the palette index of the pixel at (x, y).