ci: update short description on docker hub page for emqx/emqx

[id]

Fixes EMQX-8481

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/emqx/emqx/blob/master/CONTRIBUTING.md.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/ dir
• For EMQX 4.x: appup files updated (execute scripts/update-appup.sh emqx)
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• In case of non-backward compatible changes, reviewer should check this item as a write-off, and add details in Backward Compatibility section

Backward Compatibility

More information