Fix update_synapse_for_appservice

[nathanael-h]

Problem

• Bridge register fails at the update_synapse_forservice.sh script, and inside at chown to a non existing system user cf.https://ci-apps-dev.yunohost.org/ci/job/15106

Solution

• Update the script

PR Status

• [X] Code finished and ready to be reviewed/tested
• [X] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[Josue-T]

Thanks for the fix