Documentation: FW_HEADER_MAGIC isn't a magic number

It's a file format identifier

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/1b21808fb399931eb44f0dc26fda20a632ecc196.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


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