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
The `->findLink('Downloads')->click();` part can be replaced with just `->clickLink('Downloads');` if I'm not mistaken.