doc/build-helpers: fix appimage example where postExtract is used

The argument passed to `extract` can also contain a `postExtract` attribute, which allows you to execute additional commands after the files are extracted from the AppImage.

`postExtract` must be a string with commands to run.

:::{.warning}

When specifying `postExtract`, you should use `appimageTools.wrapAppImage` instead of `appimageTools.wrapType2`.

Otherwise `wrapType2` will extract the AppImage contents without respecting the `postExtract` instructions.

:::

:::{.example #ex-extracting-appimage-with-postextract}

# Extracting an AppImage to install extra files, using `postExtract`

This is a rewrite of [](#ex-extracting-appimage) to use `postExtract`.

This is a rewrite of [](#ex-extracting-appimage) to use `postExtract` and `wrapAppImage`.

```nix

{ appimageTools, fetchurl }:

    '';

  };

in

appimageTools.wrapType2 {

  inherit pname version src;

appimageTools.wrapAppImage {

  inherit pname version;

  src = appimageContents;

  extraPkgs = pkgs: [ pkgs.at-spi2-core ];

    install -m 444 -D ${appimageContents}/usr/share/icons/hicolor/512x512/apps/irccloud.png \

      $out/share/icons/hicolor/512x512/apps/irccloud.png

  '';

  # specify src archive for nix-update

  passthru.src = src;

}

```