Update rest-routing.md

Added a note that pluralization of controller IDs in RESTful endpoints by yii\rest\UrlRule doesn't always simply add "s" to the end of the controller ID.  I cited an example of controllers whose IDs end in the letter "x", for example "BoxController", whose RESTful endpoints are pluralized to "boxes"
Merge pull request #5272 from cnanders/patch-1

[skip ci] Update rest-routing.md