Update doctest links from onqtam/doctest to doctest/doctest

[larouxn]

Recently noticed while reading the unit testing docs here that the links to the doctest repo are still referring to the old (pre-rename?) organization/user repo onqtam/doctest. While the links aren't broken, I think it would be preferable to have the links go directly to the current doctest/doctest repo. Hence, I've updated the three references in the godot-docs repo, which are all on the unit testing page.

[larouxn]

Seeing an unrelated failure in CI. Appears the site in question may be blocking CI from accessing.

### Errors in tutorials/ui/bbcode_in_richtextlabel.rst

* [403] [https://www.computerhope.com/jargon/d/dropcap.htm](https://www.computerhope.com/jargon/d/dropcap.htm) | Network error: Forbidden

Error: Process completed with exit code 2.

[tetrapod00]

Don't worry about that CI error. It's unrelated to this PR, and has been happening for a little while on other PRs.

[skyace65]

Thanks! And congrats on your first merged PR!