docs: fix shell command with escaped brackets in pip install (#6464)

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.

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## Why are these changes needed?

[AS-IS]

![image](https://github.com/user-attachments/assets/d6c8c95a-eb45-4206-a147-cd74aa6e949f)

[After fixed]

![image](https://github.com/user-attachments/assets/f2ffea4b-f993-4888-8559-c19e2b5c6eea)


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## Related issue number

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## Checks

- [x] 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.
- [x] I've made sure all auto checks have passed.

Co-authored-by: Victor Dibia <victordibia@microsoft.com>