Documentation: NBD_REQUEST_MAGIC isn't a magic number

It's part of the line protocol, same as in commit 82805818
("Documentation: NBD_REPLY_MAGIC isn't a magic number")

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/20220927003727.slf4ofb7dgum6apt@tarta.nabijaczleweli.xyz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>