Fix mix docs build failure (#632)

* fix mix docs build failure hopefully

* Apply suggestion from @polvalente

---------

Co-authored-by: Paulo Valente <16843419+polvalente@users.noreply.github.com>

Author: aphillipo

mix.exs

@@ -2,14 +2,15 @@ defmodule Axon.MixProject do
   use Mix.Project
 
   @source_url "https://github.com/elixir-nx/axon"
-  @version "0.9.0"
+  @version "0.8.1"
 
   def project do
     [
       app: :axon,
       version: @version,
       name: "Axon",
       elixir: "~> 1.13",
+      cli: cli(),
       elixirc_paths: elixirc_paths(Mix.env()),
       deps: deps(),
       docs: docs(),
@@ -20,8 +21,10 @@ defmodule Axon.MixProject do
 
   def cli do
     [
-      docs: :docs,
-      "hex.publish": :docs
+      preferred_envs: [
+        docs: :docs,
+        "hex.publish": :docs
+      ]
     ]
   end