add AvatarURL field to api groups (in modules/structs package)

templates/swagger/v1_json.tmpl

@@ -26436,6 +26436,10 @@
       "description": "Group represents a group of repositories and subgroups in an organization",
       "type": "object",
       "properties": {
+        "avatar_url": {
+          "type": "string",
+          "x-go-name": "AvatarURL"
+        },
         "description": {
           "type": "string",
           "x-go-name": "Description"