docs: fix non-compiling Rust external-transport example (#2142)

The Rust tab of "Connecting the SDK to the external server"
constructed `Transport::External { host, port }` and omitted the
variant's third field, so the example failed to compile:

    error[E0063]: missing field `connection_token` in initializer of `github_copilot_sdk::Transport`

`connection_token` was added to the variant after the example was
written. `None` is the value that matches this section: the server
it tells you to start (`copilot --headless --port 4321`) has no
token, and the other Rust example that constructs this variant, in
docs/setup/multi-tenancy.md, already passes `None`.

Co-authored-by: examon <examon@users.noreply.github.com>

Author: examon

docs/getting-started.md

@@ -2053,6 +2053,7 @@ let mut options = ClientOptions::default();
 options.transport = Transport::External {
     host: "localhost".to_string(),
     port: 4321,
+    connection_token: None,
 };
 let client = Client::start(options).await?;