sysutils/loki: Fix a copy/paste mishap in comments

Several comments in loki's rc.d file referred to "grafana" where they
should have referred to "loki". Looks like a copy/paste mishap from
grafana's rc.d.

Approved by: portmgr blanket
Fixes: a96c988a2977 sysutils/loki: Add new port
Pull Request: https://github.com/freebsd/freebsd-ports/pull/466