Fix saving to VTK for Wedges

At some point a change from `MeshData_to_vtk` wasn't carried over to the specialized version for Wedges. Saving now works as expected. For example: ``` rd = RefElemData(Wedge(), 3) md = MeshData(uniform_mesh(Wedge(), 2), rd) (; x, y) = md u = @. (x + y)^2 v = @. x^2 + y^2 vtu_name = export_to_vtk(rd, md, [u, v], "uv.vtu") ```