Use correct function for joining arrays

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`

The function that should be used is `implode()` (aka `join()`).