Fix typo in Grafana dashboard

Change-Id: I97e9be0dc35e98e6c00a5ffafc6172a24edfebdd
diff --git a/heat/files/grafana_dashboards/heat_prometheus.json b/heat/files/grafana_dashboards/heat_prometheus.json
index debdd50..2c7149e 100644
--- a/heat/files/grafana_dashboards/heat_prometheus.json
+++ b/heat/files/grafana_dashboards/heat_prometheus.json
@@ -405,7 +405,7 @@
           "thresholds": [],
           "timeFrom": null,
           "timeShift": null,
-          "title": "Throuhgput",
+          "title": "Throughput",
           "tooltip": {
             "shared": true,
             "sort": 0,