Docfix for `org-structure-template-alist'.

* org.el (org-structure-template-alist): Fix docstring: the
feature is not experimental anymore.

lisp/org.el

@@ -11060,8 +11060,7 @@ of the `?` in the template.
 There are two templates for each key, the first uses the original Org syntax,
 the second uses Emacs Muse-like syntax tags.  These Muse-like tags become
 the default when the /org-mtags.el/ module has been loaded.  See also the
-variable `org-mtags-prefer-muse-templates'.
-This is an experimental feature, it is undecided if it is going to stay in."
+variable `org-mtags-prefer-muse-templates'."
   :group 'org-completion
   :type '(repeat
 	  (string :tag "Key")