[MatL] PM/Permeability; Fix doxygen warning.

Remove specific file name in \file macro to avoid matching
it against other/new Permeability.h.

MaterialLib/PorousMedium/Permeability/Permeability.h

 /**
+ * \file
  * \copyright
  * Copyright (c) 2012-2019, OpenGeoSys Community (http://www.opengeosys.org)
  *            Distributed under a Modified BSD License.
  *              See accompanying file LICENSE.txt or
  *              http://www.opengeosys.org/project/license
- *
- * \file:   Permeability.h
- *
  */
 
 #pragma once