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>

Documentation/x86/boot.txt

@@ -308,7 +308,7 @@ Protocol:	2.00+
 
 Field name:	start_sys
 Type:		read
-Offset/size:	0x20c/4
+Offset/size:	0x20c/2
 Protocol:	2.00+
 
   The load low segment (0x1000).  Obsolete.