fix an error in the comment.
# :outvar catkin_INCLUDE_DIRS: contains the include dirs of all searched components.
# For use with CMake ``include_directories(${catkin_INCLUDE_DIRS})``.
# :outvar catkin_LIBRARY_DIRS: contains the library dirs of all searched components.
-# For use with CMake ``link_directories(${catkin_INCLUDE_DIRS})``.
+# For use with CMake ``link_directories(${catkin_LIBRARY_DIRS})``.
# :outvar catkin_LIBRARIES: contains the libraries of all searched components.
# For use with CMake ``target_link_libraries(<target> ${catkin_LIBRARIES})``.
# :outvar <comp>_INCLUDE_DIRS/_LIBRARY_DIRS/_LIBRARIES:
