Fix export annotation being formatted as Godot 3.

While browsing the docs, I noticed that there was a bit of code obviously misformatted for Godot 4. Fixed it, hopefully.

tutorials/best_practices/logic_preferences.rst

@@ -70,7 +70,7 @@ either? Let's see an example:
     #
     # 4. It is when one instantiates this script on its own with .new() that
     #    one will load "office.tscn" rather than the exported value.
-    export(PackedScene) var a_building = preload("office.tscn")
+    @export var a_building : PackedScene = preload("office.tscn")
 
     # Uh oh! This results in an error!
     # One must assign constant values to constants. Because `load` performs a