Fixed one spelling mistake.

(cherry picked from commit b6c61785c49a81db6255fcdbecc0036bf7846d30)

tutorials/ui/gui_using_theme_editor.rst

@@ -84,7 +84,7 @@ control opens it for editing on the right-hand side.
 Theme types and items
 ~~~~~~~~~~~~~~~~~~~~~
 
-The right-hand side of them theme editor provides a list of theme types available
+The right-hand side of the theme editor provides a list of theme types available
 in the edited theme resource, and the contents of the selected type. The list of
 type's items is divided into several tabs, corresponding to each data type available
 in the theme (colors, constants, styles, etc.). If the **Show Default** option is