FIG. 1 is a perspective view of a screwdriver bit showing my new design; FIG. 2 is a front elevational view thereof, a rear elevational view being identical; FIG. 3 is a left side elevational view thereof; FIG. 4 is a right side elevational view thereof; and, FIG. 5 is a top plan view thereof, a bottom plan view being identical. The broken lines are shown for illustrative purposes only and form no part of claimed design.