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>

Documentation/x86/boot.txt

@@ -349,7 +349,7 @@ Protocol:	2.00+
 	3  SYSLINUX
 	4  EtherBoot
 	5  ELILO
-	7  GRuB
+	7  GRUB
 	8  U-BOOT
 	9  Xen
 	A  Gujin