make button field final

src/main/java/com/example/tictactoe_game/TicTacToeFX.java

@@ -8,7 +8,7 @@ import javafx.stage.Stage;
 
 public class TicTacToeFX extends Application {
 
-    private Button[][] buttons = new Button[3][3];
+    private final Button[][] buttons = new Button[3][3];
 
     private boolean isXTurn = true;