Add blank lines to improve code style. (#1329)

## Overview
Spaces have been added to the short-term-memory.mdx example code.

## Type of change

**Type:** Update existing documentation

## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [x] I have used **root relative** paths for internal links
- [x] 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 -->

Author: wangtao2001

src/oss/langchain/short-term-memory.mdx

@@ -699,6 +699,7 @@ def greet(
     """Use this to greet the user once you found their info."""
     user_name = runtime.state["user_name"]
     return f"Hello {user_name}!"
+
 agent = create_agent(
     model="gpt-5-nano",
     tools=[update_user_info, greet],