The ornamental design for a hub, as shown and described.
FIG. 1 is a front perspective view of a hub showing our new design.
FIG. 2 is a top side view thereof.
FIG. 3 is a front side view thereof.
FIG. 4 is a left side view thereof.
FIG. 5 is a right side view thereof.
FIG. 6 is a rear side view thereof; and,
FIG. 7 is a bottom side view thereof.
The broken line showing of structural features are included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.