The ornamental design for bed sheet, as shown and described.
FIG. 1 is a perspective view of a bed sheet;
FIG. 2 is an enlarged fragmentary bottom view of the bed sheet in the “installed” position;
FIG. 3 is an enlarged fragmentary perspective view of the bed sheet in the “installed” position;
FIG. 4 is an enlarged fragmentary bottom view of the inner side of the bed sheet in the “uninstalled” position, showing the elasticized band such that the attachment of the straps to the elasticized band can be seen; and,
FIG. 5 is an enlarged fragmentary bottom view of the bed sheet in the uninstalled position.
Portions of FIGS. 1-5 that are shown as broken lines are for illustrative purpose only and form no part of the claimed design.