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 : Perspective view disclosing the claim in an environment of use
The broken lines shown in reproductions 1.3 and 1.7 depict portions of the skin treatment apparatus that form no part of the claimed design. The additional broken lines in reproduction 1.8 are unclaimed environment and form no part of the claimed design.