Update "type auth.Challenge" comment example code

This interface was changed in a0fdfb9d4d, but the comment wasn't ever updated to match.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

registry/auth/auth.go

@@ -21,7 +21,9 @@
 // 			if ctx, err := accessController.Authorized(ctx, access); err != nil {
 //				if challenge, ok := err.(auth.Challenge) {
 //					// Let the challenge write the response.
-//					challenge.ServeHTTP(w, r)
+//					challenge.SetHeaders(w)
+//					w.WriteHeader(http.StatusUnauthorized)
+//					return
 //				} else {
 //					// Some other error.
 //				}