Documentation: update kernel stack for x86_64

Commit 6538b8ea ("x86_64: expand kernel stack to 16K")
expanded kernel stack for x86_64 but left the wrong documentation,
update it.

Signed-off-by: Yan Yan <yanyan.yan@antgroup.com>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Link: https://lore.kernel.org/r/20230328072537.20188-1-yanyan.yan@antgroup.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>