org-export: Use (org-version) for creator string

* contrib/lisp/org-export.el (org-export-creator-string):
  Use (org-version) for creator string.

contrib/lisp/org-export.el

@@ -325,7 +325,7 @@ If the value is `comment' insert it as a comment."
 
 (defcustom org-export-creator-string
   (format "Generated by Org mode %s in Emacs %s."
-	  (if (boundp 'org-version) org-version "(Unknown)")
+	  (if (fboundp 'org-version) (org-version) "(Unknown)")
 	  emacs-version)
   "String to insert at the end of the generated document."
   :group 'org-export-general