Scene tree: Show functions as code (#5113)

* Scene tree: Show functions as code

These function callbacks should be shown as code for better readability.

getting_started/step_by_step/scene_tree.rst

@@ -93,7 +93,7 @@ When a node is connected, directly or indirectly, to the root
 viewport, it becomes part of the *scene tree*.
 
 This means that as explained in previous tutorials, it will get the
-_enter_tree() and _ready() callbacks (as well as _exit_tree()).
+``_enter_tree()`` and ``_ready()`` callbacks (as well as ``_exit_tree()``).
 
 .. image:: img/activescene.png