The ornamental design for a broom, as shown and described.
1. Broom
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Bottom
1.6 : Top
1.7 : Right
1.8 : Enlarged partial perspective view
The broken lines immediately adjacent to the shaded area define boundaries of the claimed design and form no part thereof. The broken lines showing the remainder of the broom represent unclaimed subject matter and form no part of the claimed design.