lycheeslicer: fix desktop file Exec path (#492563)

    comment = "All-in-one 3D slicer for Resin and Filament";

    desktopName = "LycheeSlicer";

    noDisplay = false;

    exec = "lychee";

    exec = "lycheeslicer";

    terminal = false;

    mimeTypes = [ "model/stl" ];

    categories = [ "Graphics" ];