Documentation: NBD_REPLY_MAGIC isn't a magic number

It's part of the line protocol

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/a8afed8fb4d7df2c8fb95c3fa758240b2e46cdc8.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


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