Fixed Typo!

[[Imported from SVN: r4603]]

common/singleton.hh

 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
 #ifndef DUNE_SINGLETON_HH
-#define DUEN_DINGLETON_HH
+#define DUNE_SINGLETON_HH
 /**
  * @file
  * @brief Usefull wrapper for creating singletons.