Adapt CI badge URL in README

[benegee]

@tristanmontoya observed for TrixiAtmo.jl that the CI badge did not display the actual workflow status, see trixi-framework/TrixiAtmo.jl#3 (comment)

We then found that according to the docs a different URL scheme should be used: https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/adding-a-workflow-status-badge#using-the-workflow-file-name

For Trixi.jl the current status seems to be correct, but selecting branches other than main fails, see trixi-framework/TrixiAtmo.jl#3 (comment)
Although this feature is not used in Trixi.jl's README I would propose to adapt the URL.

[github-actions]

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