[KARAF-1287] - extending-console synced with KARAF-1001

As mentioned in [this post](http://karaf.922171.n3.nabble.com/Karaf-3-0-0-Console-Extender-tp4027276p4027369.html) the documentation for Karaf 3.0.0  does not reflect the change made by KARAF-1001. This small patch corrects the chapter "Extending the console" in developers guide.