Update rendering-a-template.md

There is some divergences in the render option `into`, compared to the [api docs](http://emberjs.com/api/classes/Ember.Route.html#method_render).

I believe that the correct is "the template to render into", because `options.into` is passed to `_lookupActiveView` [here](https://github.com/emberjs/ember.js/blob/c0ce02cc66f2716b1f00d2306eccb695b1c62b16/packages/ember-routing/lib/system/route.js#L1517), and [it](https://github.com/emberjs/ember.js/blob/c470b4ded06f1fef6db9c2ca2d9d1f17203b93be/packages/ember-routing/lib/system/router.js#L198) accepts a `templateName` argument.

Close emberjs/ember.js#4367