[HttpClient][PHPDoc] Fix 2 remaining return mixed

Author: fancyweb

Response/HttplugPromise.php

@@ -49,10 +49,8 @@ public function getState(): string
 
     /**
      * {@inheritdoc}
-     *
-     * @return Psr7ResponseInterface|mixed
      */
-    public function wait($unwrap = true)
+    public function wait($unwrap = true): ?Psr7ResponseInterface
     {
         $result = $this->promise->wait($unwrap);