The ornamental design for a trigger, as shown and described.
FIG. 1 is a front top left side isometric view of a trigger showing my new design.
FIG. 2 is a front bottom left side isometric view thereof;
FIG. 3 is a rear bottom right side view thereof;
FIG. 4 is a rear top right side view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a front elevation view thereof;
FIG. 8 is a rear elevation view thereof;
FIG. 9 is a top plan view thereof; and,
FIG. 10 is a bottom plan view thereof.
The broken dash lines in the figures depict portions of the trigger that form no part of the claimed design. The broken dash-dot lines denote boundaries of the claim that form no part thereof.