Fixed the return docstring of the method

Author: Alpaca00

appium/webdriver/extensions/flutter_integration/flutter_commands.py

@@ -187,7 +187,7 @@ def get_render_tree(
             text (Optional[str]): The text of the widget to filter by.
 
         Returns:
-            str: The render tree of the current screen.
+            List[Optional[Dict]]: A list of dictionaries or None values representing the render tree.
         """
         opts = {}
         if widget_type is not None: