docs: improve readibility of benchmarks readme

[tholor]

Related Issues

• Addresses remaining review comments from https://github.com/deepset-ai/haystack/pull/2766

Checklist

• [x] I have read the contributors guidelines and the code of conduct
• [x] I have updated the related issue with new insights and changes
• [ ] I added tests that demonstrate the correct behavior of the change
• [x] I've used the conventional commit convention for my PR title
• [ ] I documented my code
• [ ] I ran pre-commit hooks and fixed any issue