Fix broken link to how-to-contribute page (#4892)

Author: slhck

pages/developers/index.md

@@ -8,6 +8,6 @@ has_children: true
 # Developers
 
 This section contains useful information for developers. Some parts are still work in progress and will be added over
-time. If you like to contribute to the dash.js project checkout out the [How to contribute](how-to-contribute/index.html)
+time. If you like to contribute to the dash.js project checkout out the [How to contribute](how-to-contribute.html)
 section.