Fix warning on assets property

```
warning: giving a binary to :assets is deprecated, please give a map from source to target instead:

    [assets: %{"assets" => "assets"}]

  (ex_doc 0.38.2) lib/ex_doc/formatter/html.ex:265: ExDoc.Formatter.HTML.generate_assets/3
  (ex_doc 0.38.2) lib/ex_doc/formatter/epub.ex:35: ExDoc.Formatter.EPUB.run/3
  (ex_doc 0.38.2) lib/mix/tasks/docs.ex:531: anonymous fn/6 in Mix.Tasks.Docs.run/3
  (elixir 1.18.3) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
  (elixir 1.18.3) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
  (ex_doc 0.38.2) lib/mix/tasks/docs.ex:530: Mix.Tasks.Docs.run/3
```

Author: agatheblues

mix.exs

@@ -14,7 +14,7 @@ defmodule BitstylesPhoenix.MixProject do
       source_url: "https://github.com/bitcrowd/bitstyles_phoenix",
       docs: [
         main: "BitstylesPhoenix",
-        assets: "assets",
+        assets: %{"assets" => "assets"},
         logo: "assets/logo.svg",
         extras: ["CHANGELOG.md", "README.md", "LICENSE.txt"],
         groups_for_modules: [