x86, doc: Fix incorrect comment about 64-bit code segment descriptors

The AMD64 Architecture Programmer's Manual Volume 2, on page
89 mentions: "If the processor is running in 64-bit mode (L=1),
the only valid setting of the D bit is 0." This matches
with what the code does.
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/r/1361825650-14031-4-git-send-email-konrad.wilk@oracle.comSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/head_64.S

@@ -49,7 +49,7 @@ L3_START_KERNEL = pud_index(__START_KERNEL_map)
 startup_64:
 
 	/*
-	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 1,
+	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0,
 	 * and someone has loaded an identity mapped page table
 	 * for us.  These identity mapped page tables map all of the
 	 * kernel pages and possibly all of memory.
@@ -146,7 +146,7 @@ ident_complete:
 	jmp secondary_startup_64
 ENTRY(secondary_startup_64)
 	/*
-	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 1,
+	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0,
 	 * and someone has loaded a mapped page table.
 	 *
 	 * %esi holds a physical pointer to real_mode_data.