x86/kbuild: Fix Documentation/ reference

x86 documentation moved.  Fix the reference.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202307271956.IxoG9X0c-lkp@intel.com/