docs(CONTRIBUTING): fix markdown link syntax

Closes #2819

Author: cvrebert

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 ## Got a question or problem?
 
-Please, do not open issues for the general support questions as we want to keep GitHub issues for bug reports and feature requests. You've got much better chances of getting your question answered on (StackOverflow)[http://stackoverflow.com/questions/tagged/angular-ui-bootstrap] where maintainers are looking at questions questions tagged with `angular-ui-bootstrap`.
+Please, do not open issues for the general support questions as we want to keep GitHub issues for bug reports and feature requests. You've got much better chances of getting your question answered on [StackOverflow](http://stackoverflow.com/questions/tagged/angular-ui-bootstrap) where maintainers are looking at questions questions tagged with `angular-ui-bootstrap`.
 
 StackOverflow is a much better place to ask questions since:
 * there are hundreds of people willing to help on StackOverflow