fix: never use a shell

examples/hello-world.php

@@ -8,17 +8,6 @@ $output = (new Command('echo'))
     ->arg('Hello, World!')
     ->output();
 
-var_dump($output);
+echo $output->stdout;
 
-// object(Nih\CommandBuilder\Output)#9 (3) {
-//   ["stdout"]=>
-//   string(14) "Hello, World!
-// "
-//   ["stderr"]=>
-//   string(0) ""
-//   ["code"]=>
-//   object(Nih\CommandBuilder\ExitStatus)#8 (1) {
-//     ["code"]=>
-//     int(0)
-//   }
-// }
+// Hello, World!