Memory leak in draw_string example

**Your Godot version:**
3.stable

**Issue description:**
The code example creates an orphan node.

The below is the corrected example which does not leak memory.
```
var control = Control.new()
var default_font = control.get_font("font")
control.queue_free()
draw_string(default_font, Vector2(64, 64), "Hello world")
```

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/classes/class_canvasitem.html#class-canvasitem-method-draw-string