We claim the ornamental design for a height-adjustable desk, as shown and described.
FIG. 1 is a top perspective view of a height-adjustable desk;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a right side elevation view thereof; and,
FIG. 8 is a top perspective view thereof with the desktop raised.
The article is shown with symbolic break lines on its top and legs, where the appearance of any portion of the article between the break lines forms no part of the claimed design.