I claim the ornamental design for concrete step, as shown and described.
FIG. 1 is a front, left side, top isometric view of the concrete step, a rear, right side, top isometric view being identical;
FIG. 2 is a left side elevational view of the concrete step, a right side elevational view being identical;
FIG. 3 is a top plan view of the concrete step;
FIG. 4 is a bottom plan view of the concrete step; and,
FIG. 5 is a front elevational view of the concrete step, a rear elevational view being identical.
The structural features depicted by broken lines have been provided to illustrate portions of the concrete step that form no part of the claimed design.