Fixed singleton error

git-svn-id: svn://svn.dre.vanderbilt.edu/DOC/Middleware/trunk/ACE@97390 ae88bc3d-4319-0410-8dbf-d08b4c9d3795
ACE_NULL_MUTEX should be ACE_Null_Mutex

git-svn-id: svn://svn.dre.vanderbilt.edu/DOC/Middleware/trunk/ACE@97391 ae88bc3d-4319-0410-8dbf-d08b4c9d3795