docs: fix stale "backend" terminology in docs/README.md

The builder-settings sentence still referenced "custom HTTP backend" from
before the HttpBackend->HttpTransport rename. Updated to "custom HTTP
transport" to match the public API and the rest of the docs.

Author: claude

docs/README.md

@@ -17,7 +17,7 @@ ApifyClient client = ApifyClient.create("my-api-token");
 
 `ApifyClient.create` takes the token as an explicit argument — it does not read `APIFY_TOKEN`
 automatically. Use `ApifyClient.builder()` for non-default settings (base URL, retries, timeout,
-user-agent suffix, custom HTTP backend).
+user-agent suffix, custom HTTP transport).
 
 Get your API token from the
 [Apify Console → Settings → API & Integrations](https://console.apify.com/settings/integrations).