Added umbraco 7 code example to "WebAPI Routing"

The wording of this should be changed. Generally speaking you should have access to a UrlHelper inside any controller and view since it is a property on most base classes. This should be changed to say something like:

"Generally a UrlHelper instance will be available on most base classes like Controllers and Views but In some cases you might need to create a UrlHelper instance manually. Here's an example of a way to do that:"