The ornamental design for the ironing board, as shown and described.
FIG. 1 is a top perspective view of an ironing board;
FIG. 2 is a bottom perspective view thereof in a hanging position and with the leg folded;
FIG. 3 is a left side elevational view the right side elevational view being a mirror image thereof;
FIG. 4 is a front end view thereof;
FIG. 5 is a rear end view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line depictions of the horizontal surface fragments in FIGS. 1 and 2 have been shown for the purpose of illustrating environment that forms no part of the claimed design.
The broken line showing of structural elements in the views is included for the purpose of illustrating portions of the ironing board that form no part of the claimed design.