The info in the page is confusing. The small board between the sensor and the arduino is an on-off level switcher. It provides a digital binary singnal so can't be connected to an analog pin on the arduino.
If you wish to measure the moisture level with an analogic scale, you need to eliminate that board and then use a voltage divider and an analog pin. The sketch will be also different. Everything in explained above in the thread.
You may want to read the full thread and then don't hesitate to come back with your questions.