Fixed spelling in agentchat_transform_messages.ipynb

[Andrew8xx8]

Why are these changes needed?

This PR just fixes spelling error.

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.07%. Comparing base (32fbfa2) to head (6721e25).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2234      +/-   ##
==========================================
- Coverage   37.94%   36.07%   -1.87%     
==========================================
  Files          77       77              
  Lines        7780     7780              
  Branches     1666     1666              
==========================================
- Hits         2952     2807     -145     
- Misses       4579     4734     +155     
+ Partials      249      239      -10     

Flag	Coverage Δ
unittests	36.07% <ø> (-1.86%)	⬇️

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