Attribute Element Validation Error

From the netCDF-C test DMRs:

```
$ xmllint --noout --schema xml.xsd test_vlen1.nc.dmr
test_vlen1.nc.dmr:10: Schemas validity error : Element '{http://xml.opendap.org/ns/DAP/4.0#}Attribute': This element is not expected.
test_vlen1.nc.dmr fails to validate
```

[test_vlen1.nc.dmr](https://raw.githubusercontent.com/Unidata/netcdf-c/refs/heads/main/dap4_test/rawtestfiles/test_vlen1.nc.dmr)