Docs: incorrect start command for icingadb-redis on Debian

[julianbrost]

https://icinga.com/docs/icinga-2/latest/doc/02-installation/01-Debian/#run-icinga-db-redis gives the following command to enable and start the Redis server on Debian:

systemctl enable --now icingadb-redis

However, when actually using this command, you get the following error:

Failed to enable unit: Refusing to operate on alias name or linked unit file: icingadb-redis.service

This is with version 7.0.2-1.bullseye of the icingadb-redis package where icingadb-redis.service is an alias for icingadb-redis-server.service.

Note that this step is somewhat redundant, as the package (by Debian standards) automatically enables and starts the service anyways.