Documentation: x86: fix space instead of tab in uefi doc

Signed-off-by: NFlavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/1588080745-21999-1-git-send-email-f.suligoi@asem.itSigned-off-by: NJonathan Corbet <corbet@lwn.net>