The ornamental design for a desk, as shown and described.
FIG. 1 is a perspective view of a first embodiment of a desk showing my new design;
FIG. 2 is a front orthographic view thereof;
FIG. 3 is a top orthographic view thereof;
FIG. 4 is a left side orthographic view thereof; and,
FIG. 5 is a right side orthographic view thereof.
The broken lines shown in FIGS. 1 through 5 are included for the purpose of illustrating environmental elements only and form no part of the claimed design. The four feet located at the bottom of the desk form no part of the claimed design. The rear and bottom views are omitted and form no part of the claimed designs.