ocamlPackages.async_websocket: use dune 3

pkgs/development/ocaml-modules/janestreet/0.15.nix

···
   };
 
   async_websocket = janePackage {
+    duneVersion = "3";
     pname = "async_websocket";
     hash = "16ixqfnx9jp77bvx11dlzsq0pzfpyiif60hl2q06zncyswky9xgb";
     meta.description = "A library that implements the websocket protocol on top of Async";