emit URLs as URLs not literals

[VladimirAlexiev]

describe <https://linkedpaperswithcode.com/paper/a-new-gastric-histopathology-subsize-image>

returns anyURI literals:

  fabio:hasURL "https://arxiv.org/pdf/2106.02473v6.pdf"^^xsd:anyURI;
  <https://linkedpaperswithcode.com/property/hasURLAbstract> "https://arxiv.org/abs/2106.02473v6"^^xsd:anyURI;

The only case where this is better than real URLs is if these may be invalid URLs (eg with spaces or unescaped special chars).
But this is not the case for arxiv: so please change them to real URLs.