Update defining-your-routes.md to reflect RFC #774

https://rfcs.emberjs.com/id/0774-implicit-record-route-loading

Author: kategengler

guides/release/routing/defining-your-routes.md

@@ -286,9 +286,7 @@ Router.map(function() {
 
 If the user navigates to `/post/5`, the route will then have the `post_id` of
 `5` to use to load the correct post.
-Ember follows the convention of `:model-name_id` for two reasons.
-The first reason is that Routes know how to fetch the right model by default, if you follow the convention.
-The second is that `params` is an object, and can only have one value associated with a key.
+Ember follows the convention of `:model-name_id` because `params` is an object, and can only have one value associated with a key.
 To put it in code, the following will _not_ work properly:
 
 ```javascript {data-filename=app/router.js}