speakup: Fix typo in documentation "boo" -> "boot"

There is a typo in the speakup documentation. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20211028182319.613315-1-colin.i.king@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>