Fixed Headings regex. Match any header (h1 to h6)

Because RTL decided to switch headings of h2 to h4, some plugins were broken.
To prevent this, match any heading from <h1> to <h6> instead of hardcoded <h2>

See more info e.g. here:
http://www.xbmcnerds.com/index.php?page=Thread&postID=173351#post173351