org.el: Fix bug in done headline fontification.

* org.el (org-set-font-lock-defaults): Fix bug in done
headline fontification.

Thanks to Brian J. Carlson for pointing this error and
a solution.

lisp/org.el

@@ -5792,7 +5792,7 @@ needs to be inserted at a specific position in the font-lock sequence.")
 	   (if org-fontify-done-headline
 	       (list (format org-heading-keyword-regexp-format
 			     (concat
-			      "\\("
+			      "\\(?:"
 			      (mapconcat 'regexp-quote org-done-keywords "\\|")
 			      "\\)"))
 		     '(2 'org-headline-done t))