tutorial: make the copyright notice disappear in the HTML

IMHO it is not useful there.

tutorials/tutorial3.cpp

@@ -1,4 +1,5 @@
-/*
+/*!
+\cond SKIP
   Copyright 2012 SINTEF ICT, Applied Mathematics.
 
   This file is part of the Open Porous Media project (OPM).
@@ -15,9 +16,8 @@
 
   You should have received a copy of the GNU General Public License
   along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+\endcond
 */
-
-
 #if HAVE_CONFIG_H
 #include "config.h"
 #endif // HAVE_CONFIG_H