Remove unneeded leading slash in namespace name in usage docs

It is optional, but for consistency let's remove it.

docs/5.x/usage.md

@@ -84,7 +84,7 @@ $request = Laminas\Diactoros\ServerRequestFactory::fromGlobals(
     $_SERVER, $_GET, $_POST, $_COOKIE, $_FILES
 );
 
-$responseFactory = new \Laminas\Diactoros\ResponseFactory();
+$responseFactory = new Laminas\Diactoros\ResponseFactory();
 
 $strategy = new League\Route\Strategy\JsonStrategy($responseFactory);
 $router   = (new League\Route\Router)->setStrategy($strategy);