v3(docs): Add label_selector query param to route bindings request

[#174252451](https://www.pivotaltracker.com/story/show/174252451)

Author: pivotal-marcela-campo

docs/v3/source/includes/experimental_resources/service_route_bindings/_list.md.erb

@@ -34,6 +34,7 @@ Name | Type | Description
 **route_guids** | _list of strings_ | Comma-delimited list of route guids to filter by
 **service_instance_guids** | _list of strings_ | Comma-delimited list of service instance guids to filter by
 **service_instance_names** | _list of strings_ | Comma-delimited list of service instance names to filter by
+**label_selector** | _string_ | A query string containing a list of [label selector](#labels-and-selectors) requirements
 **include** | _list of strings_ | Optionally include a list of unique related resources in the response. Valid values are: `route`, `service_instance`
 **page** | _integer_ | Page to display; valid values are integers >= 1
 **per_page** | _integer_ | Number of results per page; <br>valid values are 1 through 5000