leds: documentation: 'ide-disk' to 'disk-activity'

Signed-off-by: Stephan Linz <linz@li-pro.net>
Cc: Joseph Jezak <josejx@gentoo.org>
Cc: Jörg Sommer <joerg@alea.gnuu.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>