Add root namespace to root namespace function

When I was trying to use Whoops - I managed to typo the editor - and was thrown back this error:-

( ! ) Fatal error: Call to undefined function Whoops\Handler\array_join() in /vagrant/vendor/filp/whoops/src/Whoops/Handler/PrettyPageHandler.php on line 201


I added a \ into the file to make this namespaced correctly.  You could alternatively

`use \array_join`