fixed issue with special chars

use [string lengthOfBytesUsingEncoding:NSUTF8StringEncoding] instead of sizeof(char) * (string.length + 1) because it calculates the length wrong if you have sepcial characters like äöü in your string
https://github.com/gutefrage/NSAttributedStringMarkdownParser/commit/507227ac32a30f1e61437f47a8c90525d18ede6a