Update the sim_sending_letter_template
CHANGELOG:
### Changed
- [#857](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/857) Update the sim_sending_letter_template
Related card: https://trello.com/c/ERyn8Vze/1339-modificacions-a-la-carta-denviament-de-sim
Edited by Pelayo García