Updated 'clone down' default editor shortcut

The default editor shortcut for 'clone down' is ctrl + shift + d.

The documentation said ctrl + d by mistake.

This reddit post can confirm this as well:
https://www.reddit.com/r/godot/comments/15pcew1/cant_duplicate_line_in_editor_clone_down_is/

tutorials/editor/default_key_mapping.rst

@@ -255,7 +255,7 @@ Text Editor
 +---------------------------+--------------------------+----------------------------+-------------------------------------------------+
 | Fold/Unfold Line          | :kbd:`Alt + F`           | :kbd:`Alt + F`             | ``script_text_editor/toggle_fold_line``         |
 +---------------------------+--------------------------+----------------------------+-------------------------------------------------+
-| Clone Down                | :kbd:`Ctrl + D`          | :kbd:`Cmd + Shift + C`     | ``script_text_editor/clone_down``               |
+| Clone Down                | :kbd:`Ctrl + Shift + D`  | :kbd:`Cmd + Shift + C`     | ``script_text_editor/clone_down``               |
 +---------------------------+--------------------------+----------------------------+-------------------------------------------------+
 | Complete Symbol           | :kbd:`Ctrl + Space`      | :kbd:`Ctrl + Space`        | ``script_text_editor/complete_symbol``          |
 +---------------------------+--------------------------+----------------------------+-------------------------------------------------+