fix typo (#3266)

Fix a tiny spelling mistake in the Grafana dashboard.
Literally a 1 character commit lol
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Signed-off-by: `Nikolai Patrick nikolaipatrick@wws.sa.edu.au`

Author: artichoked1

helm/dendrite/grafana_dashboards/dendrite-rev2.json

@@ -119,7 +119,7 @@
           "refId": "A"
         }
       ],
-      "title": "Registerd Users",
+      "title": "Registered Users",
       "type": "stat"
     },
     {