Schema version documentation (#35762)

* Update active_record_migrations.md

* Change year to match others

[Ken Greeff + Rafael Mendonça França]

guides/source/active_record_migrations.md

@@ -948,7 +948,7 @@ If `:ruby` is selected, then the schema is stored in `db/schema.rb`. If you look
 at this file you'll find that it looks an awful lot like one very big migration:
 
 ```ruby
-ActiveRecord::Schema.define(version: 20080906171750) do
+ActiveRecord::Schema.define(version: 2008_09_06_171750) do
   create_table "authors", force: true do |t|
     t.string   "name"
     t.datetime "created_at"