Fix cache key in shelly.yaml example

hack/shelly.yaml

@@ -5,8 +5,8 @@ mqtt:
   metric_per_topic_config:
     metric_name_regex: "shellies/(?P<deviceid>.*)/sensor/(?P<metricname>.*)"
   qos: 0
-  cache:
-    timeout: 24h
+cache:
+  timeout: 24h
 metrics:
   - prom_name: temperature
     # The name of the metric in a MQTT JSON message
@@ -40,4 +40,4 @@ metrics:
     # A map of string to string for constant labels. This labels will be attached to every prometheus metric
     const_labels:
       sensor_type: shelly
-    # The name of the metric in prometheus
+    # The name of the metric in prometheus