update github folder link (#291)

Author: Arhell

.github/CONTRIBUTING.md

@@ -3,5 +3,5 @@ Contributing to Yii2
 
 - [Report an issue](https://github.com/yiisoft/yii2/blob/master/docs/internals/report-an-issue.md)
 - [Translate documentation or messages](https://github.com/yiisoft/yii2/blob/master/docs/internals/translation-workflow.md)
-- [Give us feedback or start a design discussion](http://www.yiiframework.com/forum/index.php/forum/42-general-discussions-for-yii-20/)
+- [Give us feedback or start a design discussion](https://www.yiiframework.com/forum/index.php/forum/42-general-discussions-for-yii-20/)
 - [Contribute to the core code or fix bugs](https://github.com/yiisoft/yii2/blob/master/docs/internals/git-workflow.md)