WorkDir: Replace hard-coded platform name

The note at the end of the cmake configuration contained a hard-coded
`x86_64-slc6-gcc62-opt` platform tag. Replace this with the correct
cmake variable.

Projects/WorkDir/CMakeLists.txt

@@ -71,5 +71,5 @@ atlas_cpack_setup()
 message( STATUS "")
 message( STATUS "            In order to test your updates, please don't forget to" )
 message( STATUS "            set up the environment with e.g.:" )
-message( STATUS "            --->   source x86_64-slc6-gcc62-opt/setup.sh" )
+message( STATUS "            --->   source ${ATLAS_PLATFORM}/setup.sh" )
 message( STATUS "")