Fix incorrect return type in FBSimulatorSet.description
Fix incorrect return type in FBSimulatorSet.description

Summary: This has been here for a while but `po set` can cause the debugger to choke as it's an array not a string. It should call `description` but I guess it does something fancy with looking at the return types in the header.

Reviewed By: c-ryan747

Differential Revision: D21742456

fbshipit-source-id: 80ffe98b8d2247053fcba89158ef159d9b22aabf

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl