Fixed a typo, from 'buton' to 'button'.

classes/class_treeitem.rst

@@ -213,7 +213,7 @@ Method Descriptions
 
 - void **add_button** **(** :ref:`int<class_int>` column, :ref:`Texture<class_Texture>` button, :ref:`int<class_int>` button_idx=-1, :ref:`bool<class_bool>` disabled=false, :ref:`String<class_String>` tooltip="" **)**
 
-Adds a button with :ref:`Texture<class_Texture>` ``button`` at column ``column``. The ``button_idx`` index is used to identify the button when calling other methods. If not specified, the next available index is used, which may be retrieved by calling ``get_buton_count()`` immediately after this method. Optionally, the button can be ``disabled`` and have a ``tooltip``.
+Adds a button with :ref:`Texture<class_Texture>` ``button`` at column ``column``. The ``button_idx`` index is used to identify the button when calling other methods. If not specified, the next available index is used, which may be retrieved by calling ``get_button_count()`` immediately after this method. Optionally, the button can be ``disabled`` and have a ``tooltip``.
 
 .. _class_TreeItem_clear_custom_bg_color: