fix: invalid deployment instructions

[florrdv]

The Vercel deployment instructions include a trailing slash, which causes the origin checks in onmessage (dd2b2c59183b39903ef264616d8b3f862a7435df) to fail due to props.origin not including this trailing slash. This causes the onmessage handler to return early and none of the code there to be executed.

This PR introduces a small change to the deployment instructions to account for origin checking & updates package.json to remove the trailing slash from siteURL.