Update install guides migration example (#68)

As it may be seen in [Oban](https://hexdocs.pm/oban/installation.html)
it is recommended to set up hard version on the initial migration, so we
can ensure the ErrorTracker database matches the same schema over time.

This is important because other migrations may use our schema and if we
do not specify it we may end up with the latest schema update (version
10 for example) when the next migration was expecting version 2.

Author: odarriba

guides/Getting Started.md

@@ -53,8 +53,10 @@ 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()
-  def down, do: ErrorTracker.Migration.down()
+  def up, do: ErrorTracker.Migration.up(version: 2)
+
+  # We specify `version: 1` in `down`, to ensure we remove all migrations.
+  def down, do: ErrorTracker.Migration.down(version: 1)
 end
 ```