doc/mksyntaxgo: use standard generated Go header

Change generated commment to follow https://go.dev/s/generatedcode

Change-Id: I61e0da6583a5f8df2da63d669388cce83ad91d0d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62850
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

doc/mksyntaxgo

@@ -19,7 +19,7 @@ sam -d $out <<'!'
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// DO NOT EDIT. This file is generated by mksyntaxgo from the RE2 distribution.
+// Code generated by mksyntaxgo from the RE2 distribution. DO NOT EDIT.
 
 /*
 Package syntax parses regular expressions into parse trees and compiles