fix: update lc handbook 02 (#470)

Changes:
- Import `langchain-community` as it's required for `from
langchain.callbacks import get_openai_callback`.
- Import `numexpr` as it's required by `LLMMathChain`.
- Updated to use LCEL, rather than importing Chain classes.
- Removed LangChain Hub part at end of Notebook as it was used for
importingt the deprecated LLMathChain

## 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)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

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Co-authored-by: Siraj Aizlewood <[email protected]>

Author: jamescalam