📝 Fix StreamingResponse doc example to actually stream

Add `await asyncio.sleep(1)` to the async generator example so it
yields control back to the event loop between chunks, demonstrating
actual streaming behavior instead of returning all chunks at once.

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(1)
         yield b"some fake video bytes"