Update migration version in Getting Started

Author: odarriba

guides/Getting Started.md

@@ -56,7 +56,7 @@ Open the generated migration and call the `up` and `down` functions on `ErrorTra
 defmodule MyApp.Repo.Migrations.AddErrorTracker do
   use Ecto.Migration
 
-  def up, do: ErrorTracker.Migration.up(version: 3)
+  def up, do: ErrorTracker.Migration.up(version: 4)
 
   # We specify `version: 1` in `down`, to ensure we remove all migrations.
   def down, do: ErrorTracker.Migration.down(version: 1)