Fix typo in doc

[weijen]

Fix typo in doc

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 75.41%. Comparing base (7e1f7a7) to head (9b800b9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5628   +/-   ##
=======================================
  Coverage   75.41%   75.41%           
=======================================
  Files         171      171           
  Lines       10467    10467           
=======================================
  Hits         7894     7894           
  Misses       2573     2573           

Flag	Coverage Δ
unittests	75.41% <ø> (ø)

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