fix: typo in usage.md

[apokusin]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.25%. Comparing base (2230161) to head (7a5c3b1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6245      +/-   ##
==========================================
+ Coverage   72.97%   77.25%   +4.28%     
==========================================
  Files         306      200     -106     
  Lines       17374    14256    -3118     
  Branches      406        0     -406     
==========================================
- Hits        12679    11014    -1665     
+ Misses       4422     3242    -1180     
+ Partials      273        0     -273     

Flag	Coverage Δ
unittests	77.25% <ø> (+4.28%)	⬆️

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