x86: doc: boot.txt: fix the size of the start_sys field

The start_sys field is two bytes, not four.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
index 147bfe5..83c0033 100644
--- a/Documentation/x86/boot.txt
+++ b/Documentation/x86/boot.txt
@@ -308,7 +308,7 @@
 
 Field name:	start_sys
 Type:		read
-Offset/size:	0x20c/4
+Offset/size:	0x20c/2
 Protocol:	2.00+
 
   The load low segment (0x1000).  Obsolete.