Fix hook docstring to reflect renaming of state to org-state.

* lisp/org.el: (org-after-todo-state-change-hook): Fix docstring to
  reflect name change of state to org-state.

The renamed variable can cause user hooks added to
org-after-todo-state-change-hook to break, so it is essential to have
the correct variable name in the docstring.

lisp/org.el

@@ -2225,7 +2225,7 @@ property and include the word \"recursive\" into the value."
 (defcustom org-after-todo-state-change-hook nil
   "Hook which is run after the state of a TODO item was changed.
 The new state (a string with a TODO keyword, or nil) is available in the
-Lisp variable `state'."
+Lisp variable `org-state'."
   :group 'org-todo
   :type 'hook)