Contributing.md dead link to github help

[M-Dollinger]

Feature description

This line:

1. [Fork the project](https://help.github.com/forking/)

QGIS docs links
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo
instead

Additional context

I am unsure about which template to use, since it relates to the documentation in this repo

[nicogodet]

Feels free to open a PR with this change : simply fork this project, create a new branch and edit the file. It can all be done from github web interface for simple change like that

[uclaros]

fixed in #60693