chore: updated langchain handbook ch5 (#481)

…chain-retrieval-augmentation.ipynb (#476)

Now runs successfully and is up to date with latest LCEL methods.

## Problem

Describe the purpose of this change. What problem is being solved and
why?

## Solution

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or other resources.

## Type of Change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] 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)

## Test Plan

Describe specific steps for validating this change.

## Problem

Describe the purpose of this change. What problem is being solved and
why?

## Solution

Describe the approach you took. Link to any relevant bugs, issues, docs,
or other resources.

## Type of Change

- [ ] 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)

## Test Plan

Describe specific steps for validating this change.

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

Author: jamescalam