The ornamental design for a monitor, as shown and described.
FIG. 1 is a rear view of a monitor showing our new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a rear perspective view thereof.
The gray shading represents a metallic-looking surface. The broken lines are environmental only and form no part of the claimed design.