CONTRIBUTING: fix blog link

Author: ebidel

CONTRIBUTING.md

@@ -8,7 +8,7 @@ There are many ways to contribute to the Polymer project! We welcome and truly a
 
 Beyond Github, we try to have a variety of different lines of communication open:
 
-* [Blog](https://blog.polymer-project-org)
+* [Blog](https://blog.polymer-project.org/)
 * [Twitter](https://twitter.com/polymer)
 * [Google+ Community](https://plus.sandbox.google.com/u/0/communities/115626364525706131031?cfem=1)
 * [Mailing list](https://groups.google.com/forum/#!forum/polymer-dev)