Use actual working example in README.md

* Passing $startUrl to the constructor of `Behat\Mink\Driver\Goutte\Client` didn't work.
* example.com doesn't have a "Chat" button to click.
* When it did work, no feedback was given to indicate anything actually happened - this example will print a URL of a download page on php.net
That's the longest way to output a string. Try doing this instead:
```php
echo $session->getCurrentUrl() . PHP_EOL;
```