v13 docs link in OpenApi schema is broken

### Environment

* PostgreSQL version: 17.5
* PostgREST version: 13.0.0
* Operating system: NixOS

### Description of issue

The OpenAPI output contains an invalid link to the docs:

```
      url: https://postgrest.org/en/v"13"/references/api.html
```

Note the quotes around the version.