Fix the display of the "Errors" panel in the SLO detail dashboard

This panel actually shows the rate of errors occurring but is configured
to display this as percentunit. This results in it showing a 200% error
rate when there are actually 2 errors occurring per second. Change the
unit for this panel to reqps instead.

examples/grafana/detail.json

@@ -598,7 +598,7 @@
               }
             ]
           },
-          "unit": "percentunit"
+          "unit": "reqps"
         },
         "overrides": [
           {
@@ -715,4 +715,4 @@
   "uid": "ccssRIenz",
   "version": 1,
   "weekStart": ""
-}
+}