unigine-superposition: fix exec case in desktopItem (#392496)

  desktopItem = makeDesktopItem {

    name = "Superposition";

    exec = "Superposition";

    exec = "superposition";

    genericName = "A GPU Stress test tool from the UNIGINE";

    icon = "Superposition";

    desktopName = "Superposition Benchmark";