The ornamental design for a display apparatus, as shown and described.
FIG. 1 is a right perspective view of a display apparatus showing our new design;
FIG. 2 is a left perspective view thereof;
FIG. 3 is a front view thereof; and,
FIG. 4 is a top view thereof.
The broken lines shown in the drawings are included for purposes of illustrating environment or portions of the display apparatus and form no part of the claimed design.