Add f-string to analysing_tree_sequences.md - #326

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4.2.2, actions/setup-python@v5.4.0, styfle/cancel-workflow-action@0.12.1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/