fix StreamingResponse example

Fixes #14680

docs/en/docs/advanced/custom-response.md

@@ -217,7 +217,7 @@ You can also use the `status_code` parameter combined with the `response_class`
 
 Takes an async generator or a normal generator/iterator and streams the response body.
 
-{* ../../docs_src/custom_response/tutorial007_py39.py hl[2,14] *}
+{* ../../docs_src/custom_response/tutorial007_py39.py hl[4,17] *}
 
 #### Using `StreamingResponse` with file-like objects { #using-streamingresponse-with-file-like-objects }
 

docs_src/custom_response/tutorial007_py39.py

@@ -1,3 +1,5 @@
+import asyncio
+
 from fastapi import FastAPI
 from fastapi.responses import StreamingResponse
 
@@ -7,6 +9,7 @@ app = FastAPI()
 async def fake_video_streamer():
     for i in range(10):
         yield b"some fake video bytes"
+        await asyncio.sleep(0.1)
 
 
 @app.get("/")