The ornamental design for a petbed, as shown and described.
FIG. 1 is a top front perspective view of a pet bed showing my new design;
FIG. 2 is a bottom rear perspective view thereof;
FIG. 3 is a left elevational view thereof;
FIG. 4 is a right elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a front elevational view thereof; and,
FIG. 8 is a back elevational view thereof.
The broken line showing the base in FIGS. 1-8 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.