Update systemd unit's description to be more understandable

Signed-off-by: Christopher Meng <i@cicku.me>

packaging/deb/systemd/grafana-server.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=Starts and stops a single grafana instance on this system
+Description=Grafana instance
 Documentation=http://docs.grafana.org
 Wants=network-online.target
 After=network-online.target

packaging/rpm/systemd/grafana-server.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=Starts and stops a single grafana instance on this system
+Description=Grafana instance
 Documentation=http://docs.grafana.org
 Wants=network-online.target
 After=network-online.target