Fix customization group for `org-completion-use-ido'.

lisp/org.el

@@ -2340,7 +2340,7 @@ Use customize to modify this, or restart Emacs after changing it."
 
 (defcustom org-completion-use-ido nil
   "Non-ni means, use ido completion wherever possible."
-  :group 'org
+  :group 'org-completion
   :type 'boolean)  
 
 (defcustom org-completion-fallback-command 'hippie-expand