inserted through the pivot sleeve 430 and the positioning holder 40 is rotatably mounted on the pivot sleeve 430 without direct contacting the control shaft 20 avoiding the improper friction due to contact between the control shaft 20 and the positioning holder 40 the adjusting apparatus 50 includes an adjusting element 51 and a locking element 52 screwed to the adjusting element 51 the adjusting element 51 is a screw and has a disc shaped head portion 510 and a threaded adjusting shaft 511 which extends from a center of the head portion 510 and is smaller in outer diameter than the head portion 510 the locking element 52 is in the form of a column and formed with an arc peripheral surface 520 opposite the through hole 441 in the arc peripheral surface 520 is formed a threaded hole 521 in alignment with the adjusting shaft 511 the adjusting apparatus 50 is assembled in such a manner that the adjusting shaft 511 of the adjusting element 51 is inserted through the insertion hole 331 of the fixing portion 33 of the fixed holder 30 and the through hole 441 of the positioning portion 44 of the positioning folder 40 and.