update documentation

Author: defreng

docs/resources/group_access_token.md

@@ -46,7 +46,7 @@ resource "gitlab_group_variable" "example" {
 
 ### Optional
 
-- `access_level` (String) The access level for the group access token. Valid values are: `guest`, `reporter`, `developer`, `maintainer`.
+- `access_level` (String) The access level for the group access token. Valid values are: `guest`, `reporter`, `developer`, `maintainer`, `owner`.
 - `expires_at` (String) The token expires at midnight UTC on that date. The date must be in the format YYYY-MM-DD. Default is never.
 - `id` (String) The ID of this resource.