Fixed typo

Changed `prpy.util.CopyTrajectory` to `util.CopyTrajectory` to reflect @mkoval's change to the import.

Author: Siddhartha Srinivasa

src/prpy/planning/vectorfield.py

@@ -244,7 +244,7 @@ def FollowVectorField(self, robot, fn_vectorfield, fn_terminate,
 
                         status = fn_terminate()
                         if status == Status.CACHE_AND_CONTINUE:
-                            cached_traj = prpy.util.CopyTrajectory(qtraj)
+                            cached_traj = util.CopyTrajectory(qtraj)
                         if status == Status.TERMINATE:
                             break