go/types: use std "DO NOT EDIT" comment for generated hilbert test

For #13560.

Change-Id: I884e63f89d0756ca87b8c2092b4fd8360f791a2f
Reviewed-on: https://go-review.googlesource.com/39171
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/types/hilbert_test.go

@@ -50,8 +50,7 @@ func TestHilbert(t *testing.T) {
 func program(n int, out string) []byte {
 	var g gen
 
-	g.p(`// WARNING: GENERATED FILE - DO NOT MODIFY MANUALLY!
-// (To generate, in go/types directory: go test -run=Hilbert -H=%d -out=%q)
+	g.p(`// Code generated by: go test -run=Hilbert -H=%d -out=%q. DO NOT EDIT.
 
 // This program tests arbitrary precision constant arithmetic
 // by generating the constant elements of a Hilbert matrix H,