docs: Fix classification notebook small mistake (#32636)
Fix some minor issues in the Classification Notebook.
While some code still using hardcoded OpenAI model instead of selected
chat model.
Specifically, on page [Classify Text into
Labels](https://python.langchain.com/docs/tutorials/classification/)
We selected chat model before and have init_chat_model with our chosen
mode.
<img width="1262" height="576" alt="image"
src="https://github.com/user-attachments/assets/14eb436b-d2ef-4074-96d8-71640a13c0f7"
/>
But the following sample code still uses the hard-coded OpenAI model,
which in my case is obviously unrunable (lack of openai api key)
<img width="1263" height="543" alt="image"
src="https://github.com/user-attachments/assets/d13846aa-1c4b-4dee-b9c1-c66570ba3461"
/>
