Remove pinned ppxlib version conflicting with OCaml >= 4.13
Remove pinned ppxlib version conflicting with OCaml >= 4.13

These install instructions were recently updated to state that they should work with OCaml versions prior to 4.14.x, but the ppxlib version was pinned to 0.22.0 which requires OCaml <4.13. This commit simply removes the pinned version. F* builds successfully for me using the latest ppxlib.
FStarLang/FStar,FStarLang/FStar,FStarLang/FStar