use illuna:desertgravel and default:desert_stone for caves_ocean biome

mapgen.lua

@@ -80,7 +80,7 @@ add_biome("caves_dunes", "default:desert_sand", "default:sandstone", 6, "air", 8
 add_biome("caves", nil, "default:desert_stone", 4, "air", 8,
 	nil, nil, nil, nil, nil, 4, 31, 10, 2, ethereal.caves)
 
-add_biome("caves_ocean", "default:gravel", "default:sand", 3, "air", 8,
+add_biome("caves_ocean", "illuna:desertgravel", "default:desert_stone", 3, "air", 8,
 	nil, nil, nil, nil, nil, -14, 3, 10, 2, ethereal.caves)
 
 add_biome("caves_coldtop", nil, "ethereal:cold_dirt", 1, "air", 6,