Fix typo in Swarm doc

[weijen]

Fix typo in Swarm 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://microsoft.github.io/autogen/docs/Contribute#documentation 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 78.13%. Comparing base (07c5dc7) to head (86d7e5a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5435      +/-   ##
==========================================
+ Coverage   78.12%   78.13%   +0.01%     
==========================================
  Files         158      158              
  Lines        9579     9579              
==========================================
+ Hits         7484     7485       +1     
+ Misses       2095     2094       -1     

Flag	Coverage Δ
unittests	78.13% <ø> (+0.01%)	⬆️

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