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

[rhamzeh]

Thanks @Code2Life!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.45%. Comparing base (0887f1c) to head (cc299ce).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   25.45%   25.45%           
=======================================
  Files         102      102           
  Lines       10576    10576           
=======================================
  Hits         2692     2692           
  Misses       7612     7612           
  Partials      272      272           

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