The ornamental design for the dog doorknob cover, as shown and described.
FIG. 1 is a top right perspective view of the dog doorknob cover of my design;
FIG. 2 is a top right rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a right elevation view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is an exploded top left perspective view thereof, shown with environment.
The broken line showing of a dog doorknob cover is for the purpose of illustrating portions of the article and forms no part of the claimed design. The broken line showing of a doorknob and guide lines is for the purpose of illustrating environmental structure and forms no part of the claimed design.