[Book] Small reconfigure of the headings in chapter 4. VTABLEs are defined on a class, so I'm putting them in the section with namespaces and methods.
[Book] Small reconfigure of the headings in chapter 4. VTABLEs are defined on a class, so I'm putting them in the section with namespaces and methods.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@35457 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot