Add missed docblock for the parameter in the DownloadManager constructor

src/Composer/Downloader/DownloadManager.php

@@ -31,6 +31,7 @@ class DownloadManager
      * Initializes download manager.
      *
      * @param bool             $preferSource prefer downloading from source
+     * @param Filesystem|null  $filesystem   custom Filesystem object
      */
     public function __construct($preferSource = false, Filesystem $filesystem = null)
     {