An adjustment device includes a body, an adjustment inner gear ring, and an adjustment shaft. The body has a cavity therein, has a first end provided with a first hole and a second hole, and has a peripheral wall provided with a third hole. The first hole, the second hole, and the third hole are connected with the cavity. The adjustment inner gear ring is fitted in the cavity, and has a peripheral wall provided with adjustment holes. The adjustment holes have different cross-sectional areas. A second end of the adjustment shaft has an adjustment gear and passes through the second hole into the cavity. The adjustment gear meshes with the adjustment inner gear ring. The adjustment shaft is rotatable to drive rotation of the adjustment inner gear ring so as to enable the third hole to be connected with the cavity through one of the adjustment holes.