Simplifying the observable "get" example

The "get" example referenced a method on "this" called "getEach",
which is not a method of Ember.Object.

Furthermore the signature used with "getEach" in the example does not
exist in the framework.  This can be confusing to new users.
Merge pull request #3615 from corsen2000/computed-get-doc-fix

Simplifying / correcting the observable "get" example