The ornamental design for a skintreatment apparatus, as shown and described.
1. Skin treatment apparatus
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
1.8 : Reference
The broken line showing of the three circular elements in figures 1.1, 1.2, 1.5 and 1.8 depict portions of the skin treatment apparatus that form no part of the claimed design. The broken line showing of a stand in figure 1.8 depicts environment that forms no part of the claimed design.