erp5_ods_style: fix the way to get form id
if i have a url like: https://erp5xxx/form_idxxx?relative_item_relative_url=item_module/7817
with the old way, form id will be 7817
if i have a url like: https://erp5xxx/form_idxxx?relative_item_relative_url=item_module/7817
with the old way, form id will be 7817