The plate spring 141 protrudes toward the first fastening hole of the fixing bracket 110 and is inserted in the insertion groove 175 of the connecting bracket (150, see FIG. 2A), thereby being able to prevent the connecting bracket (150, see FIG. 2A) from being separated from the fixing bracket 110.

In the detailed shape of the plate spring 141, the plate spring 141 may have an inserting portion 141 having a hole to be able to be fitted on the bolt 143, a bending portion 141b extending from the inserting portion 141a and having a protrusion with a predetermined radius, and a fixing portion 141c extending from the bending portion 141b to protrude toward the first fastening groove 121 or to be inserted in the second fixing groove 133.

The inserting portion 141a of the plate spring 141 is formed to correspond to the position of the first fixing groove 131 and can be seated on the locking step 135 formed at the joint between the first fixing groove 131 and the second fixing groove 133. The plate spring 141 may bend such that the fixing portion 141c can be seated in the second fixing groove 133. That is, the entire shape may be an L-shape.