docs: fix typos in gaokao_math and hellobench dataset READMEs

[bajiu-bajiu]

Motivation

Three spelling errors in dataset READMEs. These files sit under opencompass/configs/, which is excluded from the codespell pre-commit hook, so typos in this tree are not caught automatically.

Modification

• opencompass/configs/datasets/gaokao_math/README.md — We recommand → We recommend
• opencompass/configs/datasets/subjective/hellobench/README.md — comprehenvise → comprehensive, and Detailed instructions to evalute → evaluate in a heading

Documentation only. No code, config, or dataset behaviour is affected.

BC-breaking (Optional)

No.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues. (codespell and mdformat with the repo's configured plugins both run clean on the two touched files; mdformat produces no reformatting.)
• Bug fixes are fully covered by unit tests — N/A, documentation only.
• The modification is covered by complete unit tests — N/A, documentation only.
• The documentation has been modified accordingly.