fix: Updating learn\generation\langchain\handbook\10-langchain-multi-query.ipynb

[Siraj-Aizlewood]

Updated to be more in line with modern LangChain practices, including using LCEL.

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Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [x] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

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