a main body, a top of the main body defining a recess and an annular sidewall enclosing the recess, the annular sidewall having a sliding recess extending along the circumference of an inner surface thereof;

a rotation-proof component defining a rotation-proof portion received in the sliding recess, the rotation-proof component defining an extending portion of substantially arc shape, the rotation-proof portion extending from a free end of the extending portion to show an arc shape substantially opposite to the extending portion; and

a plug rotatably and detachably mounted to the main body, the plug having a protruding portion at one side thereof, the protruding portion being capable of reciprocally sliding in the sliding recess and elastically pressing the rotation-proof portion for passing through the rotation-proof portion and then being restrained from reverse rotation, the extending portion being adapted to cover the protruding portion for cooperating with the rotation-proof portion to restrain the protruding portion from reverse rotation.