Update ex_doc compatibility

Author: rrrene

mix.exs

@@ -59,7 +59,7 @@ defmodule Credo.Mixfile do
       main: "overview",
       logo: "assets/credo-logo-with-trail.png",
       extra_section: "GUIDES",
-      assets: "guides/assets",
+      assets: %{"guides/assets" => "doc/assets"},
       formatters: ["html"],
       nest_modules_by_prefix: nest_modules_by_prefix(),
       groups_for_modules: groups_for_modules(),

mix.lock

@@ -1,6 +1,6 @@
 %{
   "bunt": {:hex, :bunt, "1.0.0", "081c2c665f086849e6d57900292b3a161727ab40431219529f13c4ddcf3e7a44", [:mix], [], "hexpm", "dc5f86aa08a5f6fa6b8096f0735c4e76d54ae5c9fa2c143e5a1fc7c1cd9bb6b5"},
-  "earmark_parser": {:hex, :earmark_parser, "1.4.44", "f20830dd6b5c77afe2b063777ddbbff09f9759396500cdbe7523efd58d7a339c", [:mix], [], "hexpm", "4778ac752b4701a5599215f7030989c989ffdc4f6df457c5f36938cc2d2a2750"},
+  "earmark_parser": {:hex, :earmark_parser, "1.4.45", "cba8369ab2a1342e419bc2760eec731b17be828941dcf494045d44766227e1d5", [:mix], [], "hexpm", "d3ec045bf122965db20c0bdb420e19ee1415843135327124918473feb4b328e8"},
   "ex_doc": {:hex, :ex_doc, "0.40.3", "4a972ffe64bc07dc605af487e98fc19b72a4185f55ca031b94c0552d6071c1d9", [:mix], [{:earmark_parser, "~> 1.4.44", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_c, ">= 0.1.0", [hex: :makeup_c, repo: "hexpm", optional: true]}, {:makeup_elixir, "~> 0.14 or ~> 1.0", [hex: :makeup_elixir, repo: "hexpm", optional: false]}, {:makeup_erlang, "~> 0.1 or ~> 1.0", [hex: :makeup_erlang, repo: "hexpm", optional: false]}, {:makeup_html, ">= 0.1.0", [hex: :makeup_html, repo: "hexpm", optional: true]}], "hexpm", "2756e357742fecd9749b489b85d67c9ce99c465f2e75728d9e6dc8d704b973de"},
   "file_system": {:hex, :file_system, "1.1.0", "08d232062284546c6c34426997dd7ef6ec9f8bbd090eb91780283c9016840e8f", [:mix], [], "hexpm", "bfcf81244f416871f2a2e15c1b515287faa5db9c6bcf290222206d120b3d43f6"},
   "inch_ex": {:hex, :inch_ex, "2.1.0", "0a6349fa71b89a96851da6bc4dcba304f74e23b66353fe47a939722b0a2ea539", [:mix], [{:bunt, "~> 1.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.2", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "bf6fbaac4dca8cbe95db58a213608c41f2d3fe404cc608b961ffb11c60190111"},