Update query for "Rule Eval Duration" (#6294)

This query was changed to use seconds in https://github.com/prometheus/prometheus/commit/f69e668fc468dfa2feb13603237810b8d748d87e

public/app/plugins/datasource/prometheus/dashboards/prometheus_stats.json

@@ -478,7 +478,7 @@
           "steppedLine": false,
           "targets": [
             {
-              "expr": "prometheus_evaluator_duration_milliseconds{quantile!=\"0.01\", quantile!=\"0.05\"}",
+              "expr": "prometheus_evaluator_duration_seconds{quantile!=\"0.01\", quantile!=\"0.05\"}",
               "interval": "",
               "intervalFactor": 2,
               "legendFormat": "{{quantile}}",