ox.el: German localization of 'References' section

* lisp/ox.el: Add German localization of the 'References' section

Suggested by Batien Guerry

lisp/ox.el

@@ -5488,6 +5488,7 @@ them."
      ("zh-CN" :html "代码%d " :utf-8 "代码%d "))
     ("References"
      ("fr" :ascii "References" :default "Références"))
+     ("de" :default "Quellen"))
     ("See section %s"
      ("da" :default "jævnfør afsnit %s")
      ("de" :default "siehe Abschnitt %s")