Fix On MQTT disconnect in web UI (#845)

code/html/index.html

@@ -1349,7 +1349,7 @@
             </div>
             <div class="pure-g module module-mqtt">
                 <div class="pure-u-1 pure-u-lg-1-4"><label>On MQTT disconnect</label></div>
-                <select class="pure-u-1 pure-u-lg-3-4" name="mqttOnDisc">
+                <select class="pure-u-1 pure-u-lg-3-4" name="relayOnDisc">
                     <option value="0">Don't change</option>
                     <option value="1">Turn the switch OFF</option>
                     <option value="2">Turn the switch ON</option>