Improve wording of some gdk doctor suggestions

What does this Merge Request do and why?

This MR improves the wording of some of the suggestions that appear in the output of gdk doctor to make the commands work regardless of the current working directory and looks like:

$ gdk doctor
.................
⚠️  WARNING: Your GDK may need attention.

asdf
================================================================================
The following asdf software is installed, but doesn't need to be:

redis 6.0.14

You can uninstall the software above by running:

  cd /Users/ash/src/gitlab-development-kit && rake asdf:uninstall_unnecessary_software

Database Migrations
================================================================================
There are pending database migrations.  To update your database, run:

  cd /Users/ash/src/gitlab-development-kit/gitlab && bundle exec rails db:migrate

GitLab
================================================================================
Your gitlab/log/ directory is 354MB.  You can truncate the log files if you wish
by running:

  cd /Users/ash/src/gitlab-development-kit && rake gitlab:truncate_logs

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.