- Updated wizard help text to refer to the actual button label

The Wizard help text did not match the buttons displayed:

![syncthing-gtk first run wizard_004](https://cloud.githubusercontent.com/assets/263427/5397013/e5f3db32-8156-11e4-8959-9ff4c92afc5d.png)

I changed the help text to reflect the button label  ("Continue") instead of ("Next")