Documentation/admin-guide: README & svga: remove use of "rdev"

Documentation/admin-guide: README & svga: remove use of "rdev"

"rdev" is considered antiquated, ancient, archaic, obsolete, deprecated
{choose any or all}.

Remove use of "rdev" and "vidmode" (a symlink to rdev) in
admin-guide/README.rst and admin-guide/svga.rst.

"rdev" was removed from util-linux in 2010:
  https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=a3e40c14651fccf18e7954f081e601389baefe3f

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Martin Mares <mj@ucw.cz>
Cc: Karel Zak <kzak@redhat.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Martin Mares <mj@ucw.cz>
Cc: linux-video@atrey.karlin.mff.cuni.cz
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20200918015640.8439-2-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>