add missing import (#1619)

Missing 'from langchain.agents.structured_output import ToolStrategy' in
the example code. Users will fail to run through the example code
without this line.

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---------

Co-authored-by: Mason Daugherty <[email protected]>

Author: dgtql

src/oss/langchain/quickstart.mdx

@@ -295,6 +295,8 @@ Let's walk through each step:
         :::python
 
         ```python
+        from langchain.agents.structured_output import ToolStrategy
+
         agent = create_agent(
             model=model,
             system_prompt=SYSTEM_PROMPT,