correct type hint for request data

Author: erinbit

src/KubernetesClient/Client.php

@@ -134,7 +134,7 @@ public function getStreamContext($verb = 'GET', $opts = [])
      * @param $endpoint
      * @param string $verb
      * @param array $params
-     * @param null $data
+     * @param mixed $data
      * @param array $options
      * @throws \Exception
      * @return bool|mixed|string