Documentation/x86/boot.txt: modify fieldname

Modify field names to the right ones:

 - start_sys was changed to start_sys_seg
 - iinitrd_addr_max was changed to ramdisk_max
 - pad2 was changed to pad2 and pad3
 - readmode_swtch was changed to realmode_swtch

Signed-off-by: Baodong Chen <[email]chenbdchenbd@gmail.com[email]>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>