Errors found in `functions_advanced`

There is a lingering `datetime.utcnow()` in the section about mutable objects as function's arguments:

```python
from datetime import datetime

def log(msg, *, dt=datetime.utcnow()):
    print(f"{dt}: {msg}")
```

Should be:
```python
from datetime import datetime, UTC


def log(msg, *, dt=datetime.now(UTC)):
    print(f"{dt}: {msg}")
```