Wrong client detection in example

[steida]

This:

if (typeof window === 'undefined') {
    throw Error('Chat should only render on the client.');
  }

Should be this:

if (typeof document === 'undefined') {
    throw Error('Chat should only render on the client.');
  }

Because Deno has window object.

[rickhanlonii]

Deno 2.0 is removing window because this pattern is way too prevalent denoland/deno#22057