Fix 'dont' in a Drupal.org URL fragment

Problem/Motivation

Follow up of 3138766: Fix "Don't" relevant typos in core

Example:
core/modules/migrate_drupal_ui/src/Form/OverviewForm.php
':url' => 'https://www.drupal.org/docs/8/upgrade/preparing-an-upgrade#dont_create_content',

@xjm reported:

I updated this URL fragment in the handbook:
https://www.drupal.org/docs/upgrading-drupal/preparing-a-site-for-upgrade-to-drupal-8#do_not_create_content

I left an <a> tag with the old name/ID in place also, so that anything else that had the old link would not be broken. Confirmed the link works as expected.

Updating that URL is also technically back portable since it's one of the placeholder values of the string, rather than part of the first argument of
t(), so it's not a string break.

Proposed resolution

':url' => 'https://www.drupal.org/docs/8/upgrade/preparing-an-upgrade#do_not_create_content',

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet