README: fix flake example

Resolves: #386

Author: t184256

README.md

@@ -192,7 +192,7 @@ A minimal example could look like the following:
   outputs = { self, nixpkgs, nix-on-droid }: {
 
     nixOnDroidConfigurations.default = nix-on-droid.lib.nixOnDroidConfiguration {
-      system = "aarch64-linux";
+      pkgs = import nixpkgs { system = "aarch64-linux"; };
       modules = [ ./nix-on-droid.nix ];
     };