Correcting xctest documentation
Correcting xctest documentation

Summary: The documentation regarding the --test-to-run argument in xctest run was incorrect, even though it was already used correctly in the test automation code

Reviewed By: zeyadsalloum

Differential Revision: D16939015

fbshipit-source-id: 3a40fb99829059cc8a28fd4e7ee5483998deaf4f

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