Fixed a bad link on "The Scala Actors API" page 

Hi, I fixed a bad link to the external http://akka.io site in the second paragraph on "The Scala Actors API" page at http://docs.scala-lang.org/overviews/core/actors.html. 

![image](https://f.cloud.github.com/assets/127138/62701/3cb5a370-5d54-11e2-9963-88fbb9f8ad5c.png)