The ornamental design for a ruler, as shown and described.
FIG. 1 is a perspective view of a ruler showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of an aperture and shelling or ribbing thereof is included for the purpose of illustrating environmental structure and forms no part of the claimed design.