ocamlPackages.ocaml_optcomp: update hash to sha256

pkgs/development/ocaml-modules/optcomp/default.nix

···
   name = "ocaml-optcomp-1.6";
   src = fetchurl {
     url = https://github.com/diml/optcomp/archive/1.6.tar.gz;
-    md5 = "d3587244dba1b8b10f24d0b60a8c700d";
-    };
+    sha256 = "0hhhb2gisah1h22zlg5iszbgqxdd7x85cwd57bd4mfkx9l7dh8jh";
+  };
   
   createFindlibDestdir = true;