docs: Update confirmation.md to add missing migration steps (#736)

* Update confirmation.md to add missing migration steps

* Update confirmation.md

---------

Co-authored-by: Zach Daniel <zachary.s.daniel@gmail.com>

documentation/tutorials/confirmation.md

@@ -56,6 +56,11 @@ defmodule MyApp.Accounts.User do
   end
 end
 ```
+Next we will have to generate and run migrations to add confirmed_at column to user resource
+
+```bash
+mix ash.codegen account_confirmation
+```
 
 Next we will define our "sender" module using `Swoosh`: