fix typo agent_chat.md

[lordlinus]

Why are these changes needed?

Typo in agent_chat.md

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 (fed9384) 30.81% compared to head (117b98b) 30.81%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1131   +/-   ##
=======================================
  Coverage   30.81%   30.81%           
=======================================
  Files          30       30           
  Lines        4037     4037           
  Branches      915      915           
=======================================
  Hits         1244     1244           
  Misses       2714     2714           
  Partials       79       79           

Flag	Coverage Δ
unittests	30.76% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.