Fix docblock example.

First arg of RulesChecker::isUnique() must be an array.

src/ORM/RulesChecker.php

@@ -299,7 +299,7 @@ public function checkDelete(EntityInterface $entity, array $options = [])
      * ### Example:
      *
      * ```
-     * $rules->add($rules->isUnique('email', 'The email should be unique'));
+     * $rules->add($rules->isUnique(['email'], 'The email should be unique'));
      * ```
      *
      * @param array $fields The list of fields to check for uniqueness.