Update custom-server docs

On the page https://blitzjs.com/docs/custom-server

the line

```
import blitz from "blitz/custom-server"
```

Results to

**Cannot find module 'blitz/custom-server' or its corresponding type declarations.ts(2307)**

Open the custom-server documentation page named in the issue and inspect the highlighted import example alongside the reported TypeScript error. Update the example to a valid form and verify that the module-resolution error no longer occurs.

Written by the indexing model from the issue text.