[doc] Fix comment on Linux/x86 boot protocol path

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/lkrnprefix.S

 
 /*
 	The following header is documented in the Linux source code at
-	Documentation/i386/boot.txt
+	Documentation/x86/boot.txt
 */
 	.org	497
 setup_sects: