Fix docstrings of a_initiate_chat

[thinkall]

Why are these changes needed?

Fix a docstring error in a_initiate_chat

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 ✅

Comparison is base (5b217c9) 34.24% compared to head (0f02c17) 36.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1539      +/-   ##
==========================================
+ Coverage   34.24%   36.87%   +2.62%     
==========================================
  Files          42       42              
  Lines        5098     5098              
  Branches     1165     1165              
==========================================
+ Hits         1746     1880     +134     
+ Misses       3209     3051     -158     
- Partials      143      167      +24     

Flag	Coverage Δ
unittests	36.87% <ø> (+2.62%)	⬆️

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[sonichi]

Thanks. I'll fix it in #1402