Auth via Manual Inline Basic Http is Misleading

On: https://getcomposer.org/doc/articles/authentication-for-private-packages.md#manual-inline-http-basic

The suggestion to use `username:password` does not work when `composer install` is initially run via a non-interactive console.

Moving the credentials to the composer.json's `config.http-basic..{username,password}` keys fixes the issue, but is non-obvious and should be called out as the preferred solution for those who wish to have credentials in their `composer.json`. If you have direction for the suggested fix in documentation, I can attempt to create a PR.