docs(readme): add missing asyncio import (#1858)

* fix missing import

* reorder imports

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Co-authored-by: Robert Craigie <robert@craigie.dev>

README.md

@@ -196,6 +196,7 @@ for chunk in stream:
 The async client uses the exact same interface.
 
 ```python
+import asyncio
 from openai import AsyncOpenAI
 
 client = AsyncOpenAI()