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/

arch/x86/Kconfig

@@ -1969,7 +1969,7 @@ config X86_USER_SHADOW_STACK
 
 	  CPUs supporting shadow stacks were first released in 2020.
 
-	  See Documentation/x86/shstk.rst for more information.
+	  See Documentation/arch/x86/shstk.rst for more information.
 
 	  If unsure, say N.