[!344] Minor update to the UGGridFactory documentation

Merge branch 'update-uggridfactory-documentation' into 'master'

ref:core/dune-grid The return value of createGrid should nowadays be captured
by a std::unique_ptr or std::shared_ptr.

See merge request [!344]

  [!344]: gitlab.dune-project.org/core/dune-grid/merge_requests/344

dune/grid/uggrid/uggridfactory.hh

@@ -135,7 +135,7 @@ namespace Dune {
      </p>
 
      <pre>
-     UGGrid<dim>* grid = factory.createGrid();
+     std::unique_ptr<UGGrid<dim> > grid = factory.createGrid();
      </pre>
 
      <p>