docs: add itemgetter in how_to/dynamic_chain (#23951)
Hello! I am honored to be able to contribute to the LangChain project
for the first time.
- **Description:** Using `RunnablePassthrough` logic without providing
`chat_history` key will result in nested keys in `question`, so I submit
a pull request to resolve this issue.
I am attaching a LangSmith screenshot below.
This is the result of the current version of the document.
<img width="1112" alt="image"
src="https://github.com/langchain-ai/langchain/assets/12846075/f0597089-c375-472f-b2bf-793baaecd836">
without `chat_history`:
<img width="1112" alt="image"
src="https://github.com/langchain-ai/langchain/assets/12846075/5c0e3ae7-3afe-417c-9132-770387f0fff2">
- **Lint and test**:
<img width="777" alt="image"
src="https://github.com/langchain-ai/langchain/assets/12846075/575d2545-3aed-4338-9779-1a0b17365418">
