The ornamental design for a cap, as shown and described.
1. Cap
1.1: Front and right side perspective view
1.2: Rear and left side perspective view
1.3: Front
1.4: Rear view
1.5: Top plan view
1.6: Bottom plan view
1.7: Right side view
1.8: Left side view
The broken lines in the drawings form no part of the claimed design; shading lines on the surface do not form part of the claimed design; the present article is made of cloth.