README: correct the source-url parameter docs (#108)

* README: correct the source-url parameter docs

* Fixup formatting

Author: grahamc

README.md

@@ -110,7 +110,7 @@ Differing from the upstream [Nix](https://github.com/NixOS/nix) installer script
 | `source-pr`             | The pull request of `nix-installer` to use (conflicts with `source-tag`, `source-revision`, and `source-branch`)                                                                                                                                                               | integer                                    |                                                                |
 | `source-revision`       | The revision of `nix-installer` to use (conflicts with `source-tag`, `source-branch`, and `source-pr`)                                                                                                                                                                         | string                                     |                                                                |
 | `source-tag`            | The tag of `nix-installer` to use (conflicts with `source-revision`, `source-branch`, `source-pr`)                                                                                                                                                                             | string                                     |                                                                |
-| `source-url`            | A URL pointing to a `nix-installer.sh` script                                                                                                                                                                                                                                  | URL                                        | `https://install.determinate.systems/nix`                      |
+| `source-url`            | A URL pointing to the `nix-installer` binary                                                                                                                                                                                                                                   | URL                                        | n/a (calculated)                                               |
 | `nix-package-url`       | The Nix package URL                                                                                                                                                                                                                                                            | URL                                        |                                                                |
 | `planner`               | The installation [planner] to use                                                                                                                                                                                                                                              | enum (`linux` or `macos`)                  |                                                                |
 | `reinstall`             | Force a reinstall if an existing installation is detected (consider backing up `/nix/store`)                                                                                                                                                                                   | Boolean                                    | `false`                                                        |