One side portion of an intermediate member 6 is inserted in one side portion of a tubular portion 71 of a fixed member 7 and rotatably coupled to the one side portion of the tubular portion 71 in this state. A vertical slit 71c is formed in the other side portion of the tubular portion 71 of a fixed member 7 so as to extend across the tubular portion 71. A lateral slit 71d that extends in a circumferential direction of the tubular portion 71 is formed at an intersection portion of the tubular portion 71 and the attachment plate 72 so as to intersect with the vertical slit 71c at a center portion of the lateral slit 71d. The vertical slit 71c and the lateral slit 71d define, as a displacement portion 71e, 71e, a portion that can be displaced to a radial direction outer side of the tubular portion 71. The displacement portion 71e is displaced to the outer side by being pressed by a pressing portion 61c of the intermediate member 6 when the intermediate member 6 is rotated to an attachment position at which substantially all of a main body 61 of the intermediate member 6 is inserted into the tubular portion 71.