Clarify what 'running a migration' means

It is not obvious to beginners that by 'running a migration' we mean executing the `bin/rails db:migrate` command.

Author: Ivan Denysov

guides/source/getting_started.md

@@ -1766,7 +1766,7 @@ and in `app/controllers/comments_controller.rb`:
     end
 ```
 
-Within the `article` model, after running a migration to add a `status` column, you would add:
+Within the `article` model, after running a migration to add a `status` column using `bin/rails db:migrate` command, you would add:
 
 ```ruby
 class Article < ApplicationRecord