Description of BagIt `fetch.txt` file is too constraining

[paulmillar]

While working on BagIt profile to support the DataCite use-case (see issue 31), @tdilauro mentioned:

The DataCite spec specifically says 'A TSV file named “fetch.txt.', whereas the BagIt spec allows spaces or tabs.

The exact definition may be found in §2.2.3 of RFC 8493.

Although this is a minor inconsistency, I think it is worth updating the DataCite description either to match the BagIt definition or to omit such details.

Cheers,
Paul.