deferedFetch -> deferredFetch

I noticed this typo when I was working with Ember Resource last week. The salient line of code is actually an [unchanged correct spelling of `deferredFetch`](https://github.com/zendesk/ember-resource/blob/fe969baaff1ee4f54e1eb9ef11dbbba02413f6df/src/ember-resource.js#L806) which will now start setting `deferredFetch` to null.

I'm currently not familiar enough to write a behavioral test that fails without this fix, but I'm happy to pair on one or dig further with some hints.

/cc @shajith @jish @dadah89