Document synchronous response.write in streaming

The Streaming section of the docs was updated to make clear that a synchronous write should be used in the callback, but this section was not updated.

docs/sanic/response.md

@@ -55,8 +55,8 @@ from sanic import response
 @app.route("/streaming")
 async def index(request):
     async def streaming_fn(response):
-        await response.write('foo')
-        await response.write('bar')
+        response.write('foo')
+        response.write('bar')
     return response.stream(streaming_fn, content_type='text/plain')
 ```