[minor] improvement over the Hashtbl doc

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14646 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
[minor] documentation typo in Hashtbl

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14647 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02