correcting the comment.

[morse-miladmohtashamirad]

looks like a "not" is missing at the end of the comment at source/include/core_mqtt_agent_config_defaults.h:

/* MQTT_AGENT_DO_NOT_USE_CUSTOM_CONFIG allows building the MQTT library
 * without a custom config. If a custom config is provided, the
 * MQTT_AGENT_DO_NOT_USE_CUSTOM_CONFIG macro should be defined. */

same goes in a few more comments.