Fix typo in Mumble section of contribute/working_together/roles/sysadmins
Originally created by @Dr_Whax on #14966 (Redmine)
Link to the Debian package tracker misses an s: https://tracker.debian.org/pkg/mumble%2Derver
should be mumble_server.
Will commit a fix.
Feature Branch: bug/14966-fix-typo-in-Mumble-section