Singularize store.find() example

This was confusing a user on StackOverflow [here](http://stackoverflow.com/questions/27178231/inflection-usage-for-fetching-model-in-emberjs).  Given that the model in question is `Post`, we should reference the singular instead of the plural version.
Merge pull request #1846 from clekstro/patch-1

Singularize store.find() example