x86_64: fix mm.txt documentation

Commit 85eb69a16aab5a394ce043c2131319eae35e6493 introduced 512 MiB
sized kernel and 1.5 GiB sized module space but omitted to change
documentation properly. Fix that.

[Wasn't the hole intentional protection hole?]

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: hpa@zytor.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>