/io.html link now located at /syncio.html should configure a 301 redirect in NGINX

https://nim-lang.org/Nim/io.html
Returns a 404 as the module has moved to lib/std/syncio

if nginx is configured with something like:
```nginx
location = /Nim/io.html {
  return 301 $scheme://$host/Nim/syncio.html;
}
```

Google will pick up the change eventually as well so it'll eventually stop showing io.html as a result and show syncio.