docs: Add typing import for CustomMiddleware (#1298)

## Overview
It might be better to add the missing import, since it’s not as commonly
imported as create_agent.

## Type of change

**Type:** [Update existing documentation]

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## Checklist
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- [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
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## Additional notes
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Author: mcavdar

src/oss/langchain/agents.mdx

@@ -690,6 +690,7 @@ Use middleware to define custom state when your custom state needs to be accesse
 ```python
 from langchain.agents import AgentState
 from langchain.agents.middleware import AgentMiddleware
+from typing import Any
 
 
 class CustomState(AgentState):