Sample in README is not working

```
>>> from fetchcode import fetch
>>> f = fetch('pkg:swift/github.com/Alamofire/Alamofire@5.4.3')
>>> f.location
'/tmp/tmp_cm02xsg'
>>> f.content_type
'application/zip'
>>> f.url
'https://github.com/Alamofire/Alamofire/archive/5.4.3.zip'
```

The sample from the README is not working

```
>>> f = fetch('pkg:swift/github.com/Alamofire/Alamofire@5.4.3')
Traceback (most recent call last):
File "", line 1, in
File "/home/thomas/vscode/fetchcode/src/fetchcode/__init__.py", line 148, in fetch
url, scheme = get_resolved_url(url, scheme)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/thomas/vscode/fetchcode/src/fetchcode/__init__.py", line 117, in get_resolved_url
url, scheme = resolution_handler(url)
^^^^^^^^^^^^^^^^^^^^^^^
File "/home/thomas/vscode/fetchcode/src/fetchcode/__init__.py", line 128, in resolve_url_from_purl
raise ValueError("Could not resolve PURL to a valid URL.")
ValueError: Could not resolve PURL to a valid URL.
```