For some reason havind an entity in an id="" field doesn't
validate fo rme. Change to a constant. It's only used internally
so it doesn't need to change with version.
git-svn-id: svn://svn.linuxfromscratch.org/BLFS/trunk/BOOK@14450 af4574ff-66df-0310-9fd7-8a98e5e911e0