Drop mention of backend selection for AsyncHTTPTransport() in docs (#2019)

There is no 'backend' parameter to AsyncHTTPTransport and it seems that
the backend is detected automatically for anyio as it is for other async
libraries.

diff --git a/docs/async.md b/docs/async.md
index 1b3506c18b9cc52d9461a304ae7c5e1aa51e688d..09d2940b15d464a205de10394a83bbe6ee63e420 100644 (file)
--- a/docs/async.md
+++ b/docs/async.md
@@ -207,12 +207,6 @@ async def main():
 anyio.run(main, backend='trio')
 ```
 
-When instantiating a transport instance explicitly, for maximum consistency, the same `backend` parameter can be passed to `AsyncHTTPTransport`.
-```python
-transport = httpx.AsyncHTTPTransport(backend="trio")
-```
-
-
 ## Calling into Python Web Apps
 
 Just as `httpx.Client` allows you to call directly into WSGI web applications,