Update third_party-roundcube.en.md

[dudemcbacon]

The ROUNDCUBEMAIL_REQUEST_PATH env var is required otherwise roundcube will not forward POST requests to the correct path.

[CodeShellDev]

There were previously issues with ROUNDCUBEMAIL_REQUEST_PATH: /rc/ not working (this is why we have to use nginx, traefik for example does not work)
not sure if these issues were fixed upstream.