Fix gdscript_exports @export_node_path example by adding quotes around class names to pass strings

tutorials/scripting/gdscript/gdscript_exports.rst

@@ -224,7 +224,7 @@ annotation:
 
 ::
 
-    @export_node_path(Button, TouchScreenButton) var some_button
+    @export_node_path("Button", "TouchScreenButton") var some_button
 
 Resources
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