[single-spa] fix public demo link (#1577)

### Description

Point the Single-SPA README demo link at the existing public deployment.
The previous `labs.vercel.dev` destination returns `404
DEPLOYMENT_NOT_FOUND`.

### Demo URL

https://vercel-microfrontends-single-spa.vercel.app/

### Type of Change

- [ ] New Example
- [x] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### Validation

On September 21, checked both destinations in a browser: the replacement
renders the composed Company application, including navigation, hero
content, and the dashboard image; the previous URL returns the Vercel
deployment-not-found page.

Author: molebox

microfrontends/single-spa/README.md

@@ -6,7 +6,7 @@
 
 This example demonstrates using [single-spa](https://single-spa.js.org/) with [Module Federation](https://module-federation.io/) on Vercel using [Vercel Microfrontends](https://vercel.com/docs/microfrontends). The application consists of two microfrontends that allows you to load multiple applications in a single page, enabling independent deployments and development.
 
-Demo URL: https://microfrontends-single-spa-module-federation-root.labs.vercel.dev/
+Demo URL: https://vercel-microfrontends-single-spa.vercel.app/
 
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