fix LockedPlanningSceneRW docs

fix the text that was originally copied from another class
(from LockedPlanningSceneRO)

it mentioned an incorrect return value type of
LockedPlanningSceneRW::operator->()
Merge pull request #400 from filipjares/hydro-devel

Fix minor typo in LockedPlanningSceneRW doxygen comment