Fixed broken URLs

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/3113355066/jobs/5047881943#step:11:560

(releasenotes/5.2.0: line 108) broken https://pillow.readthedocs.io/en/5.1.x/about.html#why-a-fork - 404 Client Error: Not Found for url: https://pillow.readthedocs.io/en/5.1.x/about.html

The link is shown at https://pillow.readthedocs.io/en/stable/releasenotes/5.2.0.html#resolve-confusion-getting-pil-pillow-version-string. It is a deliberate example of a broken URL, so I've reworded the sentence to avoid referring to it

https://github.com/python-pillow/Pillow/actions/runs/3113355066/jobs/5047881943#step:11:515

(releasenotes/versioning: line 24) broken https://github.com/python-pillow/Pillow/blob/main/RELEASING.md#point-release - Anchor 'point-release' not found
(releasenotes/versioning: line 24) broken https://github.com/python-pillow/Pillow/blob/main/RELEASING.md#embargoed-release - Anchor 'embargoed-release' not found

https://github.com/python-pillow/Pillow/actions/runs/3113355066/jobs/5047881943#step:11:518

(releasenotes/versioning: line 14) broken https://github.com/python-pillow/Pillow/blob/main/RELEASING.md#main-release - Anchor 'main-release' not found

Inspecting the headings at https://github.com/python-pillow/Pillow/blob/main/RELEASING.md, I found that the anchors actually have a prefix of 'user-content-'.

[hugovk]

Strange, the original link works when I click it and they also link to those with href="#main-release", but the anchor ID does have the prefix, id="user-content-main-release":