Use secure URLs in README (opencv#12160)

Saves a redirect from HTTP -> HTTPS each.

[ci skip]

Author: splattael

README.md

@@ -2,8 +2,8 @@
 
 ### Resources
 
-* Homepage: <http://opencv.org>
-* Docs: <http://docs.opencv.org/master/>
+* Homepage: <https://opencv.org>
+* Docs: <https://docs.opencv.org/master/>
 * Q&A forum: <http://answers.opencv.org>
 * Issue tracking: <https://github.com/opencv/opencv/issues>