Pong example scene: Set ball gravity scale to zero

So it's agnostic of the project physics settings.

https://phabricator.endlessm.com/T35515

Author: manuq

addons/block_code/examples/pong_game/pong_game.tscn

@@ -1039,6 +1039,7 @@ block_script = SubResource("Resource_52r02")
 [node name="Ball" parent="." instance=ExtResource("9_xrqll")]
 modulate = Color(0.511, 0.362, 0.972, 1)
 position = Vector2(960, 544)
+gravity_scale = 0.0
 linear_velocity = Vector2(353.553, 353.553)
 
 [node name="BlockCode" type="Node" parent="Ball"]