Bump ex_doc from 0.18.3 to 0.18.4

Bumps [ex_doc](https://github.com/elixir-lang/ex_doc) from 0.18.3 to 0.18.4.
- [Release notes](https://github.com/elixir-lang/ex_doc/releases)
- [Changelog](https://github.com/elixir-lang/ex_doc/blob/master/CHANGELOG.md)
- [Commits](elixir-lang/ex_doc@v0.18.3...v0.18.4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

mix.lock

@@ -2,8 +2,8 @@
   "certifi": {:hex, :certifi, "2.0.0", "a0c0e475107135f76b8c1d5bc7efb33cd3815cb3cf3dea7aefdd174dabead064", [], [], "hexpm"},
   "coverex": {:hex, :coverex, "1.4.15", "60fadf825a6c0439b79d1f98cdb54b6733cdd5cb1b35d15d56026c44ed15a5a8", [], [{:hackney, "~> 1.5", [hex: :hackney, repo: "hexpm", optional: false]}, {:poison, "~> 1.5 or ~> 2.0 or ~> 3.0", [hex: :poison, repo: "hexpm", optional: false]}], "hexpm"},
   "credo": {:hex, :credo, "0.8.10", "261862bb7363247762e1063713bb85df2bbd84af8d8610d1272cd9c1943bba63", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}], "hexpm"},
-  "earmark": {:hex, :earmark, "1.2.4", "99b637c62a4d65a20a9fb674b8cffb8baa771c04605a80c911c4418c69b75439", [:mix], [], "hexpm"},
-  "ex_doc": {:hex, :ex_doc, "0.18.3", "f4b0e4a2ec6f333dccf761838a4b253d75e11f714b85ae271c9ae361367897b7", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"},
+  "earmark": {:hex, :earmark, "1.2.5", "4d21980d5d2862a2e13ec3c49ad9ad783ffc7ca5769cf6ff891a4553fbaae761", [:mix], [], "hexpm"},
+  "ex_doc": {:hex, :ex_doc, "0.18.4", "4406b8891cecf1352f49975c6d554e62e4341ceb41b9338949077b0d4a97b949", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"},
   "excoveralls": {:hex, :excoveralls, "0.8.1", "0bbf67f22c7dbf7503981d21a5eef5db8bbc3cb86e70d3798e8c802c74fa5e27", [:mix], [{:exjsx, ">= 3.0.0", [hex: :exjsx, repo: "hexpm", optional: false]}, {:hackney, ">= 0.12.0", [hex: :hackney, repo: "hexpm", optional: false]}], "hexpm"},
   "exjsx": {:hex, :exjsx, "4.0.0", "60548841e0212df401e38e63c0078ec57b33e7ea49b032c796ccad8cde794b5c", [], [{:jsx, "~> 2.8.0", [hex: :jsx, repo: "hexpm", optional: false]}], "hexpm"},
   "hackney": {:hex, :hackney, "1.11.0", "4951ee019df102492dabba66a09e305f61919a8a183a7860236c0fde586134b6", [:rebar3], [{:certifi, "2.0.0", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "5.1.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "1.0.2", [hex: :mimerl, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.1", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm"},