chore: fix out-dated document of ClientTupleKey

[Code2Life]

ClientTupleKey type had changed, but readme file is still &[]ClientTupleKey

Description

References

Review Checklist

• [√] I have clicked on "allow edits by maintainers".
• [√] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [√] The correct base branch is being used, if not main
• [√] I have added tests to validate that the change in functionality is working as expected