Fix missing link

[YuanTingHsieh]

Description

Fix broken link in examples/advanced/federated-statistics/README.md

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Quick tests passed locally by running ./runtest.sh.
• [ ] In-line docstrings updated.
• [ ] Documentation updated.

[YuanTingHsieh]

/build