The ornamental design for a bookmark, as shown and described.
FIG. 1 is a front perspective view of a bookmark showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left side elevation view thereof; and,
FIG. 6 is a right side elevation view thereof.
The different types of surface shading employed in the drawings illustrates a contrasting appearance.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.