Changed Dropbox links to support direct download

[lintool]

Adding the suffix ?dl=1 is a Dropbox convention to support direct download without taking you to a landing page.

This makes it possible to check the page with our download scripts:

python scripts/check_download_links.py --url https://raw.githubusercontent.com/castorini/anserini/dropbox-links/docs/experiments-covid.md
python scripts/check_download_links.py --url https://raw.githubusercontent.com/castorini/anserini/dropbox-links/docs/experiments-covid-doc2query.md

[codecov]

Codecov Report

Merging #1422 (b03f446) into master (558ff2b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1422   +/-   ##
=========================================
  Coverage     54.79%   54.79%           
  Complexity      853      853           
=========================================
  Files           148      148           
  Lines          8595     8595           
  Branches       1218     1218           
=========================================
  Hits           4710     4710           
  Misses         3492     3492           
  Partials        393      393           

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[rodrigonogueira4]

Ha! I knew I didn't like ?dl=1 for some reason, but I couldn't remember why..

[tteofili]

+1 (I dislike ?dl=1 too)