README: convert http://kaitai.io/ to use https://`

Author: generalmimon

README.md

@@ -8,5 +8,5 @@ formats, network stream packet formats, etc.
 
 Further reading:
 
-* [About Kaitai Struct](http://kaitai.io/)
+* [About Kaitai Struct](https://kaitai.io/)
 * [About API implemented in this library](https://doc.kaitai.io/stream_api.html)