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

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

With https://github.com/encode/starlette/pull/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.