Reverse order of one-to-many relationship setup code to reflect paragraph description.

Someone please double check this as I have never used ember-data but in trying to read the docs I was very confused by this section as the code seems to be the reverse of the preceding paragraph describing the relationship.  I tried to validate this with the API docs and I think that what I have changed here is actually correct but it needs to be confirmed by someone with actual experience using ember-data.

In the very next section about defining models there is a post -> comments example that is similar to what I have here so either this example is currently wrong or in the defining models section that code is wrong.

Sorry I can't verify this myself but I don't have an ember-data project yet.