Fix an error with Craft 4.15+

Author: engram-design

src/helpers/ArrayHelper.php

@@ -49,7 +49,7 @@ public static function recursiveImplode(array $array, string $glue = ',', bool $
      * @return array
      * @link https://book.cakephp.org/4/en/core-libraries/hash.html#Cake\Utility\Hash::flatten
      */
-    public static function flatten(array $data, string $separator = '.'): array
+    public static function flatten($data, $separator = '.'): array
     {
         $result = [];
         $stack = [];