fix dashboards dir typo

Change-Id: I69c3b5202b13936a9e466dbb40a8919e35f36c0f
diff --git a/jenkins/files/grafana_dashboard/jenkins_prometheus.json b/jenkins/files/grafana_dashboards/jenkins_prometheus.json
similarity index 100%
rename from jenkins/files/grafana_dashboard/jenkins_prometheus.json
rename to jenkins/files/grafana_dashboards/jenkins_prometheus.json