x86: fix documentation bug about relocatability

This patch fixes a small bug in documentation: x86_64 also has now
the ability to build a relocatable kernel.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: vgoyal@redhat.com
Cc: kexec@lists.infradead.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>