Fix MQTT topic prefix placeholder to match actual default

Author: Dennis Braun

app/templates/setup.html

@@ -285,7 +285,7 @@ <h2>{{ t.mqtt_broker }}</h2>
                 </div>
                 <div class="form-row full">
                     <label for="mqtt_topic_prefix">{{ t.topic_prefix }}</label>
-                    <input type="text" id="mqtt_topic_prefix" name="mqtt_topic_prefix" value="{{ config.mqtt_topic_prefix }}" placeholder="fritzbox/docsis">
+                    <input type="text" id="mqtt_topic_prefix" name="mqtt_topic_prefix" value="{{ config.mqtt_topic_prefix }}" placeholder="docsight">
                 </div>
             </div>
             <div class="btn-row">