fix the comment syntax in the local cabal project file

flake.nix

@@ -59,8 +59,8 @@
       devShells = hslib.devShells {
         shellHook = ''
           cat >cabal.project.local <<EOF
-          # This file is auto-generated by the flake devShell's shellHook.  Do
-          # not edit this file.  Make changes in flake.nix.
+          -- This file is auto-generated by the flake devShell's shellHook.  Do
+          -- not edit this file.  Make changes in flake.nix.
           packages:
             .
             -- These aren't released on hackage yet so we have to get them