Default/functions: Tune speed of grass spread ABM

Through testing the chance is changed to 50 such that grass spread
speed matches that of the previous (0.4.13) ABM version.

mods/default/functions.lua

@@ -354,7 +354,7 @@ minetest.register_abm({
 		"default:snow",
 	},
 	interval = 6,
-	chance = 67,
+	chance = 50,
 	catch_up = false,
 	action = function(pos, node)
 		-- Check for darkness: night, shadow or under a light-blocking node