Update invoicing.md
Created by: BettyDolfing
Related to issue: dev/core#1436 (closed) and to PR: https://github.com/civicrm/civicrm-core/pull/16005
Created by: BettyDolfing
Related to issue: dev/core#1436 (closed) and to PR: https://github.com/civicrm/civicrm-core/pull/16005