docs: remove empty example link (vercel#32439)

https://github.com/vercel/next.js/tree/canary/examples/custom-server 
This example has been moved to the documentation


## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

docs/advanced-features/custom-server.md

@@ -7,7 +7,6 @@ description: Start a Next.js app programmatically using a custom server.
 <details>
   <summary><b>Examples</b></summary>
   <ul>
-    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server">Basic custom server</a></li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-express">Express integration</a></li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-hapi">Hapi integration</a></li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-koa">Koa integration</a></li>