Fix: Correct comment

Adding "not" to the explanation of the
MQTT_AGENT_DO_NOT_USE_CUSTOM_CONFIG macro.

#133

Author: kstribrnAmzn

source/include/core_mqtt_agent_config_defaults.h

@@ -45,7 +45,7 @@
 
 /* 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. */
+ * MQTT_AGENT_DO_NOT_USE_CUSTOM_CONFIG macro should not be defined. */
 #ifndef MQTT_AGENT_DO_NOT_USE_CUSTOM_CONFIG
     /* Include custom config file before other headers. */
     #include "core_mqtt_agent_config.h"