The ornamental design for a computer on a stick, as shown and described.
FIG. 1 is a perspective view illustrating a design for a computer on a stick;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is another side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines and the unshaded regions bounded by broken lines of the preceding FIGURES illustrate boundaries for the FIGURES; however, the broken lines and the unshaded regions bounded by broken lines form no part of the claimed design. Non-oblique line shading is meant to show contour and not surface ornamentation.