Update instructions for Dashboard.

https://www.drupal.org/project/backdrop_upgrade_status/issues/3377860

The instructions next to the Drupal 7 core "Dashboard" module are as follows:

Alternaive core module provided. No action required.

As @argiepiano points out here: https://forum.backdropcms.org/comment/5462#comment-5462 we should actually be encouraging people to disable and uninstall the Drupal 7 version of Dashboard and then enable Backdrop's version after upgrading, so that the installation code can run. Also "Alternative" is mis-spelled.