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

[Pinkbyte]

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...

[anyc]

Ah thank you. I only fixed the RC1 and forgot -9999,