Wrong anchor test server in doc

[vinamogit]

Hi,

At the end of this documentation section:
https://developers.stellar.org/docs/building-apps/connect-to-anchors/setup-for-anchored-assets/#about-stellartoml-files

This server is referenced as test server: stellar-anchor-server.herokuapp.com
It looks like this has been changed to: https://demo-wallet.stellar.org/