Fixing type in iterative linear solver example.

   Vector x( size, 1.0 );

   Vector b( size );

   matrix_ptr->vectorProduct( x, b );

   x = 0.0;

   std::cout << "Vector b = " << b << std::endl;

   /***

    * Solver the linear system

    * Solve the linear system

    */

   using LinearSolver = TNL::Solvers::Linear::GMRES< MatrixType >;

   LinearSolver solver;

   solver.setMatrix( matrix_ptr );

   x = 0.0;

   solver.solve( b, x );

   std::cout << "Vector x = " << x << std::endl;

}