Respiratory mask

FIG. 1 is a front perspective view showing a respiratory mask according to our new ornamental design;

FIG. 2 is a reverse perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a bottom view thereof;

FIG. 7 is a left side view thereof; and,

FIG. 8 is a right side view thereof.

The broken lines, shown throughout the views, represent the portions of the respiratory mask, which form no part of the claimed design.