Update observers.md

Right now I'm learning Ember and I found this example a litle bit hard to crack. If my understanding of Ember observers is correct they will only fire if the property is "consumed". In this Example this is not the case and therefore fullnameChanged is not executed. If readers without much knowledge of the underlying concepts try this out it simply won't work.
Consume observer with .on('init')
Merge pull request #1020 from Mythli/patch-1

Is it required to consume the observer in the first example?