Documentation: update kernel stack for x86_64

Commit 6538b8e ("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>

Author: yanyan0141

Documentation/arch/x86/kernel-stacks.rst

@@ -12,7 +12,7 @@ Most of the text from Keith Owens, hacked by AK
 x86_64 page size (PAGE_SIZE) is 4K.
 
 Like all other architectures, x86_64 has a kernel stack for every
-active thread.  These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big.
+active thread.  These thread stacks are THREAD_SIZE (4*PAGE_SIZE) big.
 These stacks contain useful data as long as a thread is alive or a
 zombie. While the thread is in user space the kernel stack is empty
 except for the thread_info structure at the bottom.