docs: fix typo

[RyRyRyNguyen]

Docs small correction to z_order columns argument.

Description

country should be a string in the TableOptimizer.z_order's columns argument.

Related Issue(s)

N/A

[github-actions]

ACTION NEEDED

delta-rs follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.25%. Comparing base (8c7b019) to head (18eb966).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2977   +/-   ##
=======================================
  Coverage   72.25%   72.25%           
=======================================
  Files         128      128           
  Lines       40336    40336           
  Branches    40336    40336           
=======================================
  Hits        29143    29143           
+ Misses       9335     9334    -1     
- Partials     1858     1859    +1     

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