[api] fix ep filter example

The sample request and simplified representation was incorrect.
Change the sample to match the result, the sample was used in
other spots but did not affect the other APIs.

Also fixed the sentence describing valid filters that can be
used; only `region_id` is supported, not `region` [1].

[1] e49a95ff6e/keystone/catalog/controllers.py (L484)

Change-Id: Id460b85d37acc0cba9246ace6a338a315080b10b

api-ref/source/v3-ext/ep-filter.inc

@@ -16,8 +16,8 @@ Endpoint Group
 --------------
 
 Represents a dynamic collection of service endpoints having the same
-characteristics, such as ``service_id``, ``interface``, or ``region``. Indeed,
-any endpoint attribute could be used as part of a filter.
+characteristics, such as ``service_id``, ``interface``, or ``region_id``.
+Indeed, any endpoint attribute could be used as part of a filter.
 
 A classic use case is to filter endpoints based on region. For example, suppose
 a user wants to filter service endpoints returned in the service catalog by
@@ -28,7 +28,7 @@ region, the following endpoint group may be used:
 
 This implies an Endpoint Group with filtering criteria of::
 
-  ``region_id = "1b501"``
+  region_id = "1b501"
 
 Create Association
 ==================

api-ref/source/v3-ext/samples/OS-EP-FILTER/create-endpoint-group-request.json

@@ -2,8 +2,7 @@
     "endpoint_group": {
         "description": "endpoint group description",
         "filters": {
-            "interface": "admin",
-            "service_id": "1b501"
+            "region_id": "1b501"
         }
         "name": "endpoint group name"
     }