Update PHPDocs

Author: mirko-jtl

lib/Gitlab/Api/Snippets.php

@@ -26,7 +26,7 @@ public function show($project_id, $snippet_id)
      * @param string $title
      * @param string $filename
      * @param string $code
-     * @param $visibility
+     * @param string $visibility
      * @return mixed
      */
     public function create($project_id, $title, $filename, $code, $visibility)

lib/Gitlab/Model/Project.php

@@ -937,7 +937,7 @@ public function snippets()
      * @param string $title
      * @param string $filename
      * @param string $code
-     * @param $visibility
+     * @param string $visibility
      * @return Snippet
      */
     public function createSnippet($title, $filename, $code, $visibility)