MQTT: Fix shell code block title

docs/sandbox/apps/mqtt.md

@@ -26,7 +26,7 @@ While MQTT can be set up to use a username and password, it is not recommended t
 
 To add a username and password, you will need to edit the `mosquitto.conf` file. You can find the file in the `/opt/mqtt/config/` directory. You will need to add the following lines to the file:
 
-``` shell "mosquitto.conf"
+``` shell title="mosquitto.conf"
 allow_anonymous false # (1)!
 user <username> # (2)!
 password <password> # (3)!