Fix the action status badge

[olperr1]

Please check if the PR fulfills these requirements

• [X] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Documentation fix

Does this PR introduce a new Powsybl Action implying to be implemented in simulators or pypowsybl?

• [ ] Yes, the corresponding issue is here
• [X] No

What is the current behavior?

The action status badge references an old URL.

What is the new behavior (if this is a feature change)? The action status badge references the new URL.

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [X] No

Other information:

See issue 8671 from badges/shields:

If you have a github workflow .github/workflows/test.yml name: Run Tests You need to update your badge URL from https://img.shields.io/github/workflow/status/<user>/<repo>/Run%20Tests to https://img.shields.io/github/actions/workflow/status/<user>/<repo>/test.yml?branch=main

• The key change here is that the workflow parameter is now the workflow filename, not the name: defined in yaml
• We also recommend specifying the branch param in most cases, although it is not required.