Fix broken link in readme

Description

Fixing the broken gha-matrix link in the readme, with both typo and incorrect repo 🎉

n/a

[x] The target branch is correct
- See picking the right version
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- Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
[ ] The commit messages follow our commit message guidelines
[x] The PR follows our contribution guidelines
[ ] ~Code changes follow our coding conventions~
[ ] ~This change is covered with tests (or tests aren't necessary for this change)~
[x] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
[ ] CI is green