Update deploy-a-react-site.mdx (cloudflare#23045)

fix wrong command

Author: sacru2red

src/content/docs/pages/framework-guides/deploy-a-react-site.mdx

@@ -37,7 +37,7 @@ cd my-react-app
 From here you can run your application with:
 
 ```sh
-npm start
+npm run dev
 ```
 
 <Render file="tutorials-before-you-start" />