Use localhost instead of http://localhost in TestClient(client=...) example (#2870)

With #2810, it is possible to override the client host (i.e. the remote address; see https://www.starlette.io/requests/#client-address).

The default value for `TestClient` is `testclient`, but an IP address in most circumstances.

The documentation example for overriding the client host uses a full URI, which makes no sense: it's the remote address.

Author: WilliamDEdwards

docs/testclient.md

@@ -86,7 +86,7 @@ By default, the TestClient will set the client host to `"testserver"` and the po
 You can change the client address by setting the `client` attribute of the `TestClient` instance:
 
 ```python
-client = TestClient(app, client=('http://localhost', 8000))
+client = TestClient(app, client=('localhost', 8000))
 ```
 
 ### Selecting the Async backend