x86, doc: Boot protocol 2.12 is in 3.8

The boot protocol 2.12 changes were pulled for 3.8, so update the
documentation accordingly.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

Documentation/x86/boot.txt

@@ -57,7 +57,7 @@ Protocol 2.10:	(Kernel 2.6.31) Added a protocol for relaxed alignment
 Protocol 2.11:	(Kernel 3.6) Added a field for offset of EFI handover
 		protocol entry point.
 
-Protocol 2.12:	(Kernel 3.9) Added the xloadflags field and extension fields
+Protocol 2.12:	(Kernel 3.8) Added the xloadflags field and extension fields
 	 	to struct boot_params for for loading bzImage and ramdisk
 		above 4G in 64bit.