Update getting_started.md

Add instructions for using the db:migrate:redo command after updating and existing migration file.

Author: jaredholdcroft

guides/source/getting_started.md

@@ -1728,8 +1728,13 @@ And next, let's update the database with the generated migrations:
 $ bin/rails db:migrate
 ```
 
-To choose the status for the existing articles and comments you can add a default value to the generated migration files by adding the `default: "public"` option and launch the migrations again. You can also call in a rails console `Article.update_all(status: "public")` and `Comment.update_all(status: "public")`.
+To choose the status for the existing articles and comments you can add a default value to the generated migration files by adding the `default: "public"` option. Once you have made a change to the migration file you can re-run that last step again:
 
+```bash
+$ bin/rails db:migrate:redo
+```
+
+You can also call in a rails console `Article.update_all(status: "public")` and `Comment.update_all(status: "public")`.
 
 TIP: To learn more about migrations, see [Active Record Migrations](
 active_record_migrations.html).