message: update forum link
diff: * Homepage: <http://opencv.org>
  * Docs: <http://docs.opencv.org/3.4/>
  -* Q&A forum: <http://answers.opencv.org>
  +* Q&A forum: <https://forum.opencv.org>
  + * previous forum (read only): <http://answers.opencv.org>
  * Issue tracking: <https://github.com/opencv/opencv/issues>
  ### Contributing
  