STYLE: Fix casing of CMake macro add_subdirectory

See http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Style_Guide/CMake

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21888 3bd1e089-480b-0410-8dfb-8563597acbee