DOC: Fix typo in documentation for streaming modes, correcting 'witte… (#28052)

…n' to 'written' in 'Emit custom output written using LangGraph’s
StreamWriter.'

### Changes:
- Corrected the typo in the phrase 'Emit custom output witten using
LangGraph’s StreamWriter.' to 'Emit custom output written using
LangGraph’s StreamWriter.'
- Enhanced the clarity of the documentation surrounding LangGraph’s
streaming modes, specifically around the StreamWriter functionality.
- Provided additional context and emphasis on the role of the
StreamWriter class in handling custom output.

### Issue Reference:
- GitHub issue: https://github.com/langchain-ai/langchain/issues/28051

This update addresses the issue raised regarding the incorrect spelling
and aims to improve the clarity of the streaming mode documentation for
better user understanding.

Thank you for contributing to LangChain!

- [ ] **PR title**: "package: description"
- Where "package" is whichever of langchain, community, core, etc. is
being modified. Use "docs: ..." for purely docs changes, "infra: ..."
for CI changes.
  - Example: "community: add foobar LLM"


- [ ] **PR message**:
**Description:**  
Fixed a typo in the documentation for streaming modes, changing "witten"
to "written" in the phrase "Emit custom output witten using LangGraph’s
StreamWriter."
**Issue:**  
This PR addresses and fixes the typo in the documentation referenced in
[#28051](https://github.com/langchain-ai/langchain/issues/28051).


**Issue:**  
This PR addresses and fixes the typo in the documentation referenced in
[#28051](https://github.com/langchain-ai/langchain/issues/28051).


- [ ] **Add tests and docs**: If you're adding a new integration, please
include
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.


- [ ] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

docs/docs/concepts/streaming.mdx

@@ -83,7 +83,7 @@ When using the *stream* and *astream* methods with LangGraph, you can **one or m
 - **"updates"**: Emit only the node name(s) and updates that were returned by the node(s) after each step.
 - **"debug"**: Emit debug events for each step.
 - **"messages"**: Emit LLM [messages](/docs/concepts/messages) [token-by-token](/docs/concepts/tokens).
-- **"custom"**: Emit custom output witten using [LangGraph's StreamWriter](https://langchain-ai.github.io/langgraph/reference/types/#langgraph.types.StreamWriter).
+- **"custom"**: Emit custom output written using [LangGraph's StreamWriter](https://langchain-ai.github.io/langgraph/reference/types/#langgraph.types.StreamWriter).
 
 For more information, please see:
 * [LangGraph streaming conceptual guide](https://langchain-ai.github.io/langgraph/concepts/streaming/) for more information on how to stream when working with LangGraph.