Fix function example code

[akimthedream]

I used the example code for function calling in one of my projects yesterday. It worked fine as long as there was only one item in message["tool_calls"], but when multiple items were returned, the client responded with a 400 error for the second_response. After reviewing the official function calling documentation, I found that the message containing tool_calls is expected to appear only once, not for each item individually. This small change resolves the issue.

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