docs: fix Jekyll formatting of another code block

[muzimuzhi]

Similar to change proposed in #79.

Another possibility is to drop ${{ ... }} and simply use if: <expr>, since

GitHub Actions automatically evaluates the if conditional as an expression.
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsif

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Before (https://mxschmitt.github.io/action-tmate/)

After (https://muzimuzhi.github.io/action-tmate/, temp)