I claim the ornamental design for a USB memory stick, as shown and described.
FIG. 1 is a Top, front, right side perspective view of USB memory stick, showing our new design;
FIG. 2 is a Bottom, rear, left side perspective view thereof; and,
FIG. 3 is a top, front, right side perspective view thereof, wherein the cap is removed.
The broken line showings of the connector and the cavity in the cap in FIG. 3 are included for the purpose of illustrating unclaimed portions of the USB memory stick and form no part of the claimed design.