fix: change hooks testnet url

[pdp2121]

High Level Overview of Change

The hooks testnet url was changed a while ago but the previous one worked. That seems to have been disabled recently. New url is https://xahau-test.net

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

Did you update CHANGELOG.md?

• [ ] Yes
• [x] No, this change does not impact library users

[pdp2121]

Closed because of: https://github.com/XRPLF/xrpl-py/pull/705