Change the scaffolding documentation link in README.txt for Drupal 9

Problem/Motivation

The scaffolding documentation link in README.txt refers to version Drupal 8.

see
https://git.drupalcode.org/project/drupal/tree/8.8.x/composer/Plugin/Sca...

Proposed resolution

see
https://git.drupalcode.org/project/drupal/tree/9.1.x/composer/Plugin/Sca...

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet