fix(docs): update module import for ToolNode in sql-agent.mdx (#1357)

## Overview
update module import for ToolNode in sql-agent.mdx langgraph tutorial

## Type of change

**Type:** Update existing documentation

## Related issues/PRs

## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [x] I have read the [contributing guidelines](README.md)
- [ ] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [ ] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
- I have gotten approval from the relevant reviewers
- (Internal team members only / optional) I have created a preview
deployment using the [Create Preview Branch
workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

## Additional notes
<!-- Any other information that would be helpful for reviewers -->
The code needed a update to run due to an error importing
[ToolNode](https://reference.langchain.com/python/langgraph/agents/?h=toolnode#langgraph.prebuilt.tool_node.ToolNode)

Following [guideline for quick
edit](https://docs.langchain.com/oss/python/contributing/documentation#quick-edit%3A-fix-a-typo)

Author: Deoshun

src/oss/langgraph/sql-agent.mdx

@@ -270,10 +270,10 @@ Putting these steps in dedicated nodes lets us (1) force tool-calls when needed,
 ```python
 from typing import Literal
 
-from langchain.agents import ToolNode
 from langchain.messages import AIMessage
 from langchain_core.runnables import RunnableConfig
 from langgraph.graph import END, START, MessagesState, StateGraph
+from langgraph.prebuilt import ToolNode
 
 
 get_schema_tool = next(tool for tool in tools if tool.name == "sql_db_schema")