The ornamental design for a hook rack, as shown and described.
FIG. 1 is a perspective view of a hook rack showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right side elevational view thereof, the left side elevational view being a mirror image;
FIG. 5 is a rear elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing portions of the hook rack for the purposes of illustrating environment and forms no part of the claimed design.