Change from "Team" to Team::class

To be consistent with the rest of the doc

docs/en/02_Developer_Guides/00_Model/02_Relations.md

@@ -23,7 +23,7 @@ use SilverStripe\ORM\DataObject;
 class Player extends DataObject
 {
     private static $has_one = [
-        "Team" => "Team",
+        "Team" => Team::class,
     ];
 }