make Namespace_ builder return Namespace_

Hi, just a little detail. This should fix invalid return type of docs

lib/PhpParser/Builder/Namespace_.php

@@ -37,7 +37,7 @@ class Namespace_ extends Declaration
     /**
      * Returns the built node.
      *
-     * @return Node The built node
+     * @return Stmt\Namespace_ The built node
      */
     public function getNode() : Node {
         return new Stmt\Namespace_($this->name, $this->stmts, $this->attributes);