The ornamental design for a ruler, as shown and described.
FIG. 1 is a top plan view of the ruler showing our new design;
FIG. 2 is a bottom plan view of the ruler of FIG. 1;
FIG. 3 is a front elevation view of the ruler of FIG. 1, the back elevation view being substantially similar thereto with the exception of the numerals shown best in FIG. 1; and,
FIG. 4 is a left side view of the ruler of FIG. 1, the right side view being identical thereto.