docs: fix shell command with escaped brackets in pip install

[roharon]

Fix the installation command in python/samples/agentchat_chainlit/README.md by properly escaping or quoting package names with square brackets to prevent shell interpretation errors in zsh and other shells.

Why are these changes needed?

[AS-IS]

[After fixed]

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 79.53%. Comparing base (9bbcfa0) to head (5a9346a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6464   +/-   ##
=======================================
  Coverage   79.53%   79.53%           
=======================================
  Files         225      225           
  Lines       16650    16650           
=======================================
  Hits        13242    13242           
  Misses       3408     3408           

Flag	Coverage Δ
unittests	79.53% <ø> (ø)

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