x86, boot: documentation whitespace fixup

Fix an extra space that sneaked in with commit 09c205af "(x86, boot:
Define the 2.12 bzImage boot protocol").
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/boot.txt

@@ -58,7 +58,7 @@ Protocol 2.11:	(Kernel 3.6) Added a field for offset of EFI handover
 		protocol entry point.
 
 Protocol 2.12:	(Kernel 3.8) Added the xloadflags field and extension fields
-	 	to struct boot_params for loading bzImage and ramdisk
+		to struct boot_params for loading bzImage and ramdisk
 		above 4G in 64bit.
 
 Protocol 2.13:	(Kernel 3.14) Support 32- and 64-bit flags being set in