The ornamental design for the citrus reamer, as shown and described.
FIG. 1 is a front elevation view of a citrus reamer showing our new design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a perspective view thereof.
The broken lines and the areas within them which are adjacent to the shaded features and surfaces depict portions of the citrus reamer that form no part of the claimed design.