The ornamental design for a toiletbowl covering, as shown and described.
FIG. 1 is a top plan view of a toilet bowl covering showing my new design;
FIG. 2 is a bottom plan view of the toilet bowl covering;
FIG. 3 is a side view of the toilet bowl covering;
FIG. 4 is a front view of the toilet bowl covering;
FIG. 5 is a rear view of the toilet bowl covering; and,
FIG. 6 is a top perspective view of the toilet bowl covering, the portions in phantom being for illustrative purposes only and forming no portion of the claimed invention.