Custom arXiv corpus: dummy URIs no longer working as URLs

[juhoinkinen]

Previously the URIs of form https://arxiv.org/list/<arxiv-category> had associated webpages, but that is no longer the case, they give 404 error, e.g.: https://arxiv.org/list/astro-ph.CO

I noticed this while showing the Custom Corpus Jupyter notebook during the tutorial session. This part is now false information and should be updated:

As the arXiv taxonomy does not use URIs, we create (dummy) URIs based on the category codes by prepending each category code with https://arxiv.org/list/, which makes the URIs act as URLs to pages on arXiv.org website.