Update impl/mozfetcher/_config.py

The xulrunner release for windows is at http://ftp.mozilla.org/pub/mozilla.org/xulrunner/releases/6.0.2/runtimes/xulrunner-6.0.2.en-US.win32.zip

I couldn't get through the example in the documentation because chromeless kept trying to get the xulrunner from a location where it does not exist. After changing the URL, the xulrunner package could be fetched and the example ran.