Mention deadnix in supported tools (#4528)

Deadnix support was added in
https://github.com/dense-analysis/ale/pull/4443 but it seems not to have
been mentioned in the lists of supported tools.

doc/ale-supported-languages-and-tools.txt

@@ -386,6 +386,7 @@ Notes:
   * `nimpretty`
 * nix
   * `alejandra`
+  * `deadnix`
   * `nix-instantiate`
   * `nixfmt`
   * `nixpkgs-fmt`