Use 2.3.0 instead of 2.3

dune.module

 Module: dune-grid
-Version: 2.3
+Version: 2.3.0
 Maintainer: dune-devel@dune-project.org
 Depends: dune-common (>= 2.3) dune-geometry (>= 2.3)
 Whitespace-Hook: Yes