The ornamental design for an armchair, as shown and described.
1. Armchair
1.1 is a perspective view; 1.2 is a right side view; 1.3 is a left side view; 1.4 is a front view; 1.5 is a rear view; 1.6 is a top view; 1.7 is a bottom view.
The broken lines shown in the drawings of the armchair are for the purposes of illustrating the environment and form no part of the claimed design.