Update return docblock of Mailable::render() to string

see #22639

Author: tentus

src/Illuminate/Mail/Mailable.php

@@ -167,7 +167,7 @@ public function later($delay, Queue $queue)
     /**
      * Render the mailable into a view.
      *
-     * @return \Illuminate\View\View
+     * @return string
      */
     public function render()
     {