The ornamental design for a button, as shown and described.
FIG. 1 is a front perspective view illustrating my design for a button;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof.
FIG. 5 is a left side view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a right view thereof.
The broken line showing of button holes in FIGS. 1, 2, and 6 is included for the purpose of showing environmental portions of the button and forms no part of the claimed design.