Address async docs review feedback

Author: marandaneto

README.md

@@ -44,7 +44,6 @@ from posthog import AsyncPosthog
 
 async with AsyncPosthog("<ph_project_api_key>", host="<ph_client_api_host>") as posthog:
     await posthog.capture("page_viewed", distinct_id="user_123")
-    await posthog.flush()
 ```
 
 Async feature flag evaluation uses non-blocking HTTP requests: