Fix one failure mode in the user guide

[peterychang]

Why are these changes needed?

The user guide will often fail because the LLM does not provide all the code required to solve the problem in a single response

Related issue number

Resolves #4645

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.

[peterychang]

This PR has a tab spacing diff. I can try to fix it if it matters, but if not, use this to see the actual change

https://github.com/microsoft/autogen/pull/4658/files?w=1

(just append ?w=1 to the diff URL to ignore whitespace)

[ekzhu]

Let's merge this change.

However I think this quick start isn't so quick. We should consider moving this to the multi-agent design pattern section, and update this quick start with something super simple, such as two agents counting from 10 to 1, without using any model or executor, just pure Python.