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>