Minor edit to dm-setbg to fix the "shuffle" function

This is just a very small change to dm-setbg. In the shuffle function, since find (on line 63) is already giving the full path to the wallpaper, there is no need to have the setbg_dir variable in the path when the path is being echoed (on line 64), so all I've done is removed it. Adding the setbg_dir variable to the path of the wallpaper makes the path something along the lines of /home/USER/Pictures/Wallpapers//home/USER/Pictures/Wallpapers/WALLPAPER_NAME instead of just /home/USER/Pictures/Wallpapers/WALLPAPER_NAME, which causes the script to crash when the random option is chosen due to the wrong path.