index-cz: fix charset header

This should hopefully fix the rendering problem on WP 8.1, as reported here:

https://twitter.com/MartinHujer/status/545100345069031424

![screenshot](https://pbs.twimg.com/media/B5CV397CIAAmTQU.jpg)
It may be enough to replace `charset="utf8"` with `charset="utf-8"`, but am opting to use a full Content-Type header to make sure even archaic browsers like IE6/IE7 pick it up.