Bump ex_doc from 0.18.1 to 0.18.3

Bumps [ex_doc](https://github.com/elixir-lang/ex_doc) from 0.18.1 to 0.18.3.
- [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@v0.18.1...v0.18.3)

Signed-off-by: dependabot[bot] <[email protected]>

mix.lock

@@ -1,6 +1,6 @@
 %{"certifi": {:hex, :certifi, "2.3.1", "d0f424232390bf47d82da8478022301c561cf6445b5b5fb6a84d49a9e76d2639", [:rebar3], [{:parse_trans, "3.2.0", [hex: :parse_trans, repo: "hexpm", optional: false]}], "hexpm"},
-  "earmark": {:hex, :earmark, "1.2.4", "99b637c62a4d65a20a9fb674b8cffb8baa771c04605a80c911c4418c69b75439", [:mix], [], "hexpm"},
-  "ex_doc": {:hex, :ex_doc, "0.18.1", "37c69d2ef62f24928c1f4fdc7c724ea04aecfdf500c4329185f8e3649c915baf", [:mix], [{:earmark, "~> 1.1", [repo: "hexpm", hex: :earmark, optional: false]}], "hexpm"},
+  "earmark": {:hex, :earmark, "1.2.5", "4d21980d5d2862a2e13ec3c49ad9ad783ffc7ca5769cf6ff891a4553fbaae761", [:mix], [], "hexpm"},
+  "ex_doc": {:hex, :ex_doc, "0.18.3", "f4b0e4a2ec6f333dccf761838a4b253d75e11f714b85ae271c9ae361367897b7", [: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", [:mix], [{:jsx, "~> 2.8.0", [hex: :jsx, optional: false]}]},
   "hackney": {:hex, :hackney, "1.12.1", "8bf2d0e11e722e533903fe126e14d6e7e94d9b7983ced595b75f532e04b7fdc7", [:rebar3], [{:certifi, "2.3.1", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "5.1.1", [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"},