rtx.conf update to fix ghosting (for 0.0.15) file

Simply replace the rtx.conf that comes with the 0.0.15 release of the demo mod.

If its still to dark or if you actually prefer the darker visuals:

1. Open the remix menu by pressing ALT + X
2. Rendering Tab -> Post Processing -> Tonemapping
3. Play around with Exposure Level & Shadow Level