What is claimed is the ornamental design for an umbrella hook, as shown and described.
FIG. 1 is a first perspective view of an umbrella hook showing the new claimed design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a perspective view thereof shown in an environment in which it may be used.
The broken lines immediately adjacent to the claimed portions are boundaries, all other broken lines are environmental structures; all the broken lines form no part of the claimed design.