message: 3.4 docs for 3.4 branch
diff: ### Resources
  * Homepage: <http://opencv.org>
  -* Docs: <http://docs.opencv.org/master/>
  +* Docs: <http://docs.opencv.org/3.4/>
  * Q&A forum: <http://answers.opencv.org>
  * Issue tracking: <https://github.com/opencv/opencv/issues>
  