x86, doc only: Correct real-mode kernel header offset for init_size

The real-mode kernel header init_size field is located at 0x260 per the field
listing in th e"REAL-MODE KERNEL HEADER" section. It is listed as 0x25c in
the "DETAILS OF HEADER FIELDS" section, which overlaps with pref_address.
Correct the details listing to 0x260.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Link: http://lkml.kernel.org/r/541cf88e2dfe5b8186d8b96b136d892e769a68c1.1310441260.git.dvhart@linux.intel.com
CC: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/x86/boot.txt

@@ -674,7 +674,7 @@ Protocol:	2.10+
 
 Field name:	init_size
 Type:		read
-Offset/size:	0x25c/4
+Offset/size:	0x260/4
 
   This field indicates the amount of linear contiguous memory starting
   at the kernel runtime start address that the kernel needs before it