rename Caml -> StdLib

[justjoheinz]

I am a complete ocaml newbie, hence I do not create a PR, however I tried to execute:

 dune build examples/hello_async.exe                                             [10:59:57]
File "examples/hello_async.ml", line 64, characters 50-64:
64 |     try Wm.Rd.lookup_path_info_exn "what" rd with Caml.Not_found -> "world"
                                                       ^^^^^^^^^^^^^^
Error (alert deprecated): module Core.Caml
[since 2023-01] use Stdlib instead of Caml

File "examples/hello_async.ml", line 64, characters 50-64:
64 |     try Wm.Rd.lookup_path_info_exn "what" rd with Caml.Not_found -> "world"
                                                       ^^^^^^^^^^^^^^
Error: Unbound constructor Caml.Not_found

the error was indeed fixed by replacing the moduke/namespace name Caml with StdLib inside the example/hello_aync.ml file. I do not know if there is a better or different solution to the fix.