Fix error: Could not find a matching version of package

What does this MR do?

Fix error "Could not find a matching version of package" by update index.md.

• Expectation: after created a project under a user and invited groups, we can install composer package with with group ID.
• Bug: we are not able to install composer package with group ID.
• Resolution: created a project under the group and we can install composer package as what the docs said. Updated GitLab official document to avoid misunderstand; idk it is a real bug in GitLab or just a confusion in documents.
• Proposal: fix the GitLab Packages & Registries work as expectation, like what the current docs says.

See more at this blog post which belong to php.watch https://php.watch/articles/composer-gitlab-repositories#create-group

baotd@baotd-VirtualBox:~/Downloads/new-composer$ composer require "baotd/my-composer-package": "1.0.0"

  [InvalidArgumentException]                                                   
  Could not find a matching version of package baotd/my-composer-package. Che  
  ck the package spelling, your version constraint and that the package is av  
  ailable in a stability which matches your minimum-stability (stable).