Fix(ContractService): use showed_name for product description
CHANGELOG:
### Fixed
- [#1155](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1155) Use showed_name instead of custom_name for product description in our contract service API.
Related card: https://trello.com/c/uyG2LwoV/816-revisar-cas-tarifes-compartides-ov
Edited by Gerard Funosas