Merged in plugin_101 (pull request #475)

Cosmetic change to match source code

Approved-by: Louise Poubel <lupoubel@hotmail.com>
Approved-by: Ian Chen <ichen@osrfoundation.org>

Author: iche033

plugins_hello_world/tutorial-6.md

@@ -140,7 +140,7 @@ New in `gazebo6`: c++11 flags are now required for all downstream software to co
 This is done with the following line:
 
 ~~~
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${GAZEBO_CXX_FLAGS}")
+list(APPEND CMAKE_CXX_FLAGS "${GAZEBO_CXX_FLAGS}")
 ~~~
 
 Create the build directory