The design is the visual features of the ergonomic cap shown in solid lines in the drawings, whether those features are features of one of shape, configuration, ornament or pattern or are a combination of any of these features.FIG. 1 is a perspective view of an ergonomic cap showing the new design;FIG. 2 is a front view thereof;FIG. 3 is a rear view thereof;FIG. 4 is a right side view thereof;FIG. 5 is a left side view thereof;FIG. 6 is a top view thereof; andFIG. 7 is a bottom view thereof.Drawings of the design are included.