The ornamental design for a monitor, as shown and described.
FIG. 1 is a front perspective view of a monitor showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a rear perspective view thereof.
The broken lines defining the unshaded regions represent the bounds of the claimed design while all other broken lines are directed to environment only the broken lines shown throughout form no part of the claimed design.