Fix upgrading link

Author: btecu

README.md

@@ -8,7 +8,7 @@ Lightweight, composable tree component for Ember without any dependency.
 * Ember CLI v2.13 or above
 
 If you are using 0.5.x and would like to upgrade to 0.6.0, please note there
-are BREAKING CHANGES. For details, see (Upgrading)[UPGRADING.md].
+are BREAKING CHANGES. For details, see [Upgrading](UPGRADING.md).
 
 
 ## Installation