docs: fix the gsm8k download on the reproducibility page

[Shi-Dong]

Summary

Follow-up to #2398, from the same audit of dataset download commands: the reproducibility page has the identical class of bug the Quick Start page had.

• docs/examples/reproducibility.md downloads openai/gsm8k, whose files live at main/train-00000-of-00001.parquet (and socratic/…), with only question/answer columns.
• The launcher the page then runs, examples/experimental/reproducibility/run-qwen2.5-0.5B-gsm8k.sh, expects /root/gsm8k/train.parquet and /root/gsm8k/test.parquet with --input-key messages --label-key label — so the download satisfies neither the paths nor the columns.
• zhuzilin/gsm8k serves exactly that contract: root-level train.parquet/test.parquet whose columns include messages and label (verified via the Hub tree and the datasets server). It is also the repo the example's own README (examples/experimental/reproducibility/README.md) already tells users to download — the docs page was the odd one out.

One-line change: the download command now fetches zhuzilin/gsm8k.

Test plan

• hf download --repo-type dataset zhuzilin/gsm8k --local-dir /root/gsm8k produces /root/gsm8k/train.parquet and test.parquet
• bash examples/experimental/reproducibility/run-qwen2.5-0.5B-gsm8k.sh reads prompt data without path or key errors