Use group for mushroom spread ABM (#2863)

Have mushroom spread abm use group:mushroom to spread not only default red and brown but all defined shrooms.

mods/flowers/init.lua

@@ -248,7 +248,7 @@ end
 
 minetest.register_abm({
 	label = "Mushroom spread",
-	nodenames = {"flowers:mushroom_brown", "flowers:mushroom_red"},
+	nodenames = {"group:mushroom"},
 	interval = 11,
 	chance = 150,
 	action = function(...)