The ornamental design for a ruler, as shown and described.
FIG. 1 is a front perspective elevational view of a ruler showing my new shape and design;
FIG. 2 is a front perspective elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a front elevational view thereof;
FIG. 7 is a left side elevational view thereof;
FIG. 8 is a left side elevational view thereof;
FIG. 9 is a right side elevational view thereof; and,
FIG. 10 is a right side elevational view thereof.
The broken line showing of a structural feature is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.