fix: use await asyncio.sleep() in StreamingResponse async generator example

The async generator in the StreamingResponse docs example did not await
anything, causing all chunks to be returned at once instead of streaming.
Added await asyncio.sleep(0) so the event loop can yield between chunks,
enabling proper streaming behavior.

Closes #14680

docs_src/custom_response/tutorial007_py39.py

@@ -1,3 +1,5 @@
+import asyncio
+
 from fastapi import FastAPI
 from fastapi.responses import StreamingResponse
 
@@ -6,6 +8,7 @@ app = FastAPI()
 
 async def fake_video_streamer():
     for i in range(10):
+        await asyncio.sleep(0)
         yield b"some fake video bytes"