Documentation: sndmagic.h doesn't exist

It was added in 2.5.5 and removed in a 2.6.9 "ALSA CVS update", pre-git,
which states:
   Removal and replacement of magic memory allocators and casts
   (core part)

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/09e56999b0b323fb0add61f7dbd8c9f0a576561a.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


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