Fixed some hardcoded values in Tree

src/pocketmine/level/generator/object/Tree.php

@@ -32,9 +32,9 @@ use pocketmine\utils\Random;
 abstract class Tree{
 	public $overridable = [
 		Block::AIR => true,
-		6 => true,
-		17 => true,
-		18 => true,
+		Block::SAPLING => true,
+		Block::LOG => true,
+		Block::LEAVES => true,
 		Block::SNOW_LAYER => true,
 		Block::LOG2 => true,
 		Block::LEAVES2 => true