The ornamental design for a an injection apparatus, as shown and described.
FIG. 1 is a perspective view taken from the front and left side of the injection apparatus, showing my new design;
FIG. 2 is a front elevational view of the injection apparatus, the rear elevational view being a mirror image of the front elevational view, and the top plan view and the bottom plan view being respectively identical to the front elevational view;
FIG. 3 is a right side elevational view thereof; and,