updated ocamlbuild workflow wiki page

`client_dir`, `server_dir` and `type_dir` are not relative to `_build` directory but to the dirname of each eliom files (of course, everything built with ocamlbuild is located inside the `_build` directory.
so from `a/b/c.eliom`, we'll get `_build/a/b/server/c.ml`, `_build/a/b/client/c.ml`, ...