update README

README.md

@@ -6,15 +6,16 @@ Horizon is a haskell package set targetting new builds of GHC.
 
 ## Updating the package set
 
-The package set is generated from the `manifest.dhall` using the following command:
+The package set is generated from the `horizon.dhall` using the following command:
 
 ```
-nix run .#horizon-gen-nix
+nix run .#horizon-gen-nix make-package-set
 ```
 
-This will generate both the `pkgs` folder and the `overlay.nix`. If you want to
-regenerate a particular package in `pkgs`, delete that file from `pkgs`. To
-regenerate the entire package set from scratch, delete the `pkgs` folder.
+This will generate both the `pkgs` folder and the `initial-packages.nix`. If
+you want to regenerate a particular package in `pkgs`, delete that file from
+`pkgs`. To regenerate the entire package set from scratch, delete the `pkgs`
+folder.
 
 If you need to do additional manual overrides to the nix code, such as
 `addPkgconfigDepends`, edit the `configuration.nix` overlay, which is applied