The ornamental design for a tap, as shown and described.
1.1 is a front perspective view of a tap, showing my new design;
1.2 is a left side elevation view;
1.3 is a right side elevation view;
1.4 is a top plan view;
1.5 is a front elevation view; and
1.6 is rear elevation view.
The portion of the design shown in the color black in view 1.5 of the drawings is for the purpose of illustrating unclaimed portions of the tap and forms no part of the claimed design.