The ornamental design for a ventilationduct, as shown and described.
FIG. 1 is an isometric view of a ventilation duct showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a right side view thereof; and,
FIG. 4 is a left side view thereof.
The broken lines and unshaded areas depict portions of the ventilation duct that form no part of the claimed design. The horizontal broken lines adjacent to shaded areas are features that also serve as boundaries to the claim and form no part thereof. The broken lines in FIGS. 1 and 2 within the stadium shaped features depict the boundaries of the claim and form no part thereof.