samples: net: mqtt: Use proper Kconfig option names

Use the correct Kconfig options names instead of generalized one that
does not exist.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

samples/net/mqtt/doc/architecture.rst

@@ -58,7 +58,7 @@ The following code snippet shows how a module thread polls for incoming messages
    Zbus implements internal message queues for subscribers.
    In some cases, depending on the use case, it might be necessary to increase the queue size for a particular subscriber.
    Especially if the module thread can block for some time.
-   To increase the message queue associated with a subscriber, increase the value of the corresponding Kconfig option, ``CONFIG_MQTT_SAMPLE_<MODULE_NAME>_MESSAGE_QUEUE_SIZE``.
+   To increase the message queue associated with a subscriber, increase the value of the corresponding Kconfig option, ``CONFIG_MQTT_SAMPLE_SAMPLER_MESSAGE_QUEUE_SIZE`` or ``CONFIG_MQTT_SAMPLE_TRANSPORT_MESSAGE_QUEUE_SIZE``.
 
 Modules that are setup as listeners have dedicated callbacks that are invoked every time there is a change to an observing channel.
 The difference between a listener and a subscriber is that listeners do not require a dedicated thread to process incoming messages.