Update group_milestones.md

doc/api/group_milestones.md

@@ -44,7 +44,8 @@ Example Response:
     "start_date": "2013-11-10",
     "state": "active",
     "updated_at": "2013-10-02T09:24:18Z",
-    "created_at": "2013-10-02T09:24:18Z"
+    "created_at": "2013-10-02T09:24:18Z",
+    "web_url": "https://gitlab.com/groups/gitlab-org/-/milestones/42"
   }
 ]
 ```