x86/doc: spelling fix for grub

Impact: documentation fix

I met okuji-san (GRUB maintainer) yesterday.
He said GRuB isn't correct spelled and he want to fix it.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>