The ornamental design for a coil, as shown and described.
FIG. 1 is a top side isometric view of a coil which embodies my new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a left side view thereof; and,
FIG. 8 is a right side view thereof.
The dash lines depict portions of the coil that form no part of the claimed design. The dot dash broken lines depict the boundaries of the claim and form no part thereof.