docs: fix node sdk initialization (#163)

apps/docs/get-started/nodejs.mdx

@@ -38,7 +38,14 @@ icon: node-js
     ```javascript
     import { Unsend } from "unsend";
 
-    const unsend = new Unsend({ apiKey: "us_12345" });
+    const unsend = new Unsend("us_12345");
+    ```
+
+    If you are running a self-hosted version of Unsend, pass the base URL as the
+    second argument:
+
+    ```javascript
+    const unsend = new Unsend("us_12345", "https://my-unsend-instance.com");
     ```
 
   </Step>

apps/docs/guides/use-with-react-email.mdx

@@ -53,7 +53,7 @@ import { Unsend } from "unsend";
 import { render } from "@react-email/render";
 import { Email } from "./email";
 
-const unsend = new Unsend({ apiKey: "us_your_unsend_api_key" });
+const unsend = new Unsend("us_your_unsend_api_key");
 
   const html = await render(<Email url="https://unsend.dev" />);