The ornamental design for an armrest, as shown and described.
FIG. 1 is a perspective view from the front and one side of the armrest;
FIG. 2 is an enlarged perspective view from the front and one side of the armrest;
FIG. 3 is a perspective view from the back and one side of the armrest;
FIG. 4 is a perspective view from the back and the other side of the armrest;
FIG. 5 is a top plan view of the armrest;
FIG. 6 is a side elevation view of the armrest; and,
FIG. 7 is a front elevation view of the armrest.
The broken line showing of the armrest is included for the purpose of illustrating environment and forms no part of the claimed design. The dash-dot lines, which define a boundary of the claimed design, form no part thereof.