Update system-tasks.rst (GIT8266O-819)

[AbdelwahedTahri]

'uiT' task priority is actually set to 1.

in components/esp8266/source/startup.c there is : assert(xTaskCreate(user_init_entry, "uiT", ESP_TASK_MAIN_STACK, NULL, ESP_TASK_MAIN_PRIO, NULL) == pdPASS);

and in esp_task.h there is :

define ESP_TASK_MAIN_PRIO (ESP_TASK_PRIO_MIN + 1) // (0 + 1)

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