Fix the reference to contributing.md in WGs description

Author: beyhan

toc/working-groups/WORKING-GROUPS.md

@@ -1,6 +1,6 @@
 Most community activity is organized into _working groups_.
 
-Working groups follow the [contributing](../CONTRIBUTING.md) guidelines although
+Working groups follow the [contributing](../../CONTRIBUTING.md) guidelines although
 each of these groups may operate a little differently depending on their needs
 and workflow.