Fix disk_replace_image_index() return value
Created by: jdgleaver
While updating RetroArch's disk control interface code (https://github.com/libretro/RetroArch/pull/10045) I noticed that the disk_replace_image_index()
function of this core breaks spec by always returning false
- this will make 'append disk image' operations fail.
This trivial PR just makes disk_replace_image_index()
return true
when successful.