Updating the example link used in downloader.py

[LeileiChui]

The original file link (ubuntu-20.04.3-desktop-amd64.iso) is now unavailable, now replaced with a link to the latest lts version of Ubuntu desktop (ubuntu-22.04.3-desktop-amd64.iso).

Type of changes

• [ ] Bug fix
• [ ] New feature
• [x] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [ ] I've run the latest black with default args on new code.
• [ ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [ ] I've added tests for new code.
• [x] I accept that @willmcgugan may be pedantic in the code review.

Description

Original link now is 404 Not Found, replaced with the link of latest lts version of Ubuntu desktop.

original link

$ curl --HEAD https://releases.ubuntu.com/20.04/ubuntu-20.04.3-desktop-amd64.iso
HTTP/1.1 200 Connection established

HTTP/1.1 404 Not Found
Date: Tue, 28 Nov 2023 05:58:37 GMT
Server: Apache/2.4.29 (Ubuntu)
Content-Type: text/html; charset=iso-8859-1

new link

$ curl --HEAD https://releases.ubuntu.com/22.04/ubuntu-22.04.3-desktop-amd64.iso
HTTP/1.1 200 Connection established

HTTP/1.1 200 OK
Date: Tue, 28 Nov 2023 06:00:32 GMT
Server: Apache/2.4.29 (Ubuntu)
Last-Modified: Tue, 08 Aug 2023 01:19:36 GMT
ETag: "12c44a000-6025f27ffd9c2"
Accept-Ranges: bytes
Content-Length: 5037662208
Content-Type: application/x-iso9660-image