The ornamental designs for a height-adjustable leg, as shown and described.
FIG. 1 is a front, top and right perspective view of a height-adjustable leg of our new design;
FIG. 2 is a rear, bottom and left perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a right side view thereof, a left side view being a mirror image thereof; and,
FIG. 8 is a reference perspective view showing a state that the height-adjustable legs are attached to four corners on a bottom plate of a base on which a person can get.
The broken lines showing of additional height adjustable legs and platform structure is for illustrative purposes only and forms no part of the claimed design.