media-libs/libsdl: fix DOCS variable to reflect upstream rename of some files

without this fix, ebuild libsdl-2.0.0_pre9999.ebuild clean merge fails with:

!!! dodoc: BUGS does not exist
 * ERROR: media-libs/libsdl-2.0.0_pre9999 failed (install phase):
 *   dodoc failed
 *
 * If you need support, post the output of `emerge --info '=media-libs/libsdl-2.0.0_pre9999'`,
 * the complete build log and the output of `emerge -pqv '=media-libs/libsdl-2.0.0_pre9999'`.
 * This ebuild is from an overlay named 'steam-overlay': '/home/pinkbyte/dev/steam-overlay/'
 * The complete build log is located at '/home/tmp/portage/media-libs/libsdl-2.0.0_pre9999/temp/build.log'.
 * The ebuild environment file is located at '/home/tmp/portage/media-libs/libsdl-2.0.0_pre9999/temp/environment'.
 * Working directory: '/home/tmp/portage/media-libs/libsdl-2.0.0_pre9999/work/libsdl-2.0.0_pre9999'
 * S: '/home/tmp/portage/media-libs/libsdl-2.0.0_pre9999/work/libsdl-2.0.0_pre9999'
 * QA Notice: file does not exist:
 *
 *      dodoc: BUGS does not exist

Note that BUGS is not only file, that was renamed, details is in commit...