Fix minor typos in AgentChat and Core python documentation

[Den-Dre]

Why are these changes needed?

This PR fixes minor typos in the AgentChat and Core python documentation to improve clarity.

Related issue number

N/A

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.
• (N/A) I've added tests (if relevant) corresponding to the changes introduced in this PR.
• (N/A) I've made sure all auto checks have passed.

[Den-Dre]

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