Fix url bibtex to support opened in Brave Browser

[yasirroni]

The old link in http://jmlr.org/papers/v21/20-091.html seems to be dead and moved to https://www.jmlr.org/papers/v21/20-091.html

[rtavenar]

On my side, both links work...

[yasirroni]

Ah, I see.

I was previously tested the link in brave. Tested in chrome, the older link works.

I don't think brave strictly only allow HTTPS because AFAIK, it will only warn user when going to HTTP.
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Tested in brave, it return

Check your Internet connection
Check any cables and reboot any routers, modems or other network devices you may be using.

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I don't know the cause. If you accept this PR, than it will support both browser. If this PR is rejected, I'm okay with it since the old link works on certain browser.

[yasirroni]

I test brave to open HTTP (http://example.com/) and it works.

Then, it might not be because brave didn't support HTTP, but the old link might has other issue.