Update CONTRIBUTING.md

Author: JLJu

CONTRIBUTING.md

@@ -1,3 +1,3 @@
 [Check the code of conduct here](/CODE_OF_CONDUCT.md)
 ## Best way of contribution is to find some open issue or bug and fix it!
-[General contribution and contact info](http://www.basicairdata.eu/join/)
+[General contribution and contact info](https://www.basicairdata.eu/about/)