grafana service: unbreak

Accidentally broken by 4fede53c0996938979d2966a69f108782a8c1c12
("nixos manuals: bring back package references").

Without this fix, grafana won't start:

$ systemctl status grafana
...
systemd[1]: Starting Grafana Service Daemon...
systemd[1]: Started Grafana Service Daemon.
grafana[666]: 2016/03/06 19:57:32 [log.go:75 Fatal()] [E] Failed to detect generated css or javascript files in static root (%!s(MISSING)), have you executed default grunt task?
systemd[1]: grafana.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: grafana.service: Unit entered failed state.
systemd[1]: grafana.service: Failed with result 'exit-code'.

Changed files
+1 -1
nixos
modules
services
monitoring
+1 -1
nixos/modules/services/monitoring/grafana.nix
···
staticRootPath = mkOption {
description = "Root path for static assets.";
-
default = "${cfg.package.out}/share/grafana/public";
+
default = "${cfg.package}/share/grafana/public";
type = types.str;
};