fix: gh pages publish dir

[marc0olo]

currently https://dfinity.github.io/stable-structures/html needs to be accessed instead of the in the readme advertised URL https://dfinity.github.io/stable-structures

I expect this change to fix this.

[github-actions]

canbench 🏋 (dir: .) 82056c4 2025-05-22 12:13:02 UTC

✅ ./canbench_results.yml is up to date
📦 canbench_results_benchmark.csv available in artifacts

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Summary:
  instructions:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

  heap_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

  stable_memory_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

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CSV results saved to canbench_results.csv