Bump ex_doc from 0.19.3 to 0.20.2 (#345)

Bumps [ex_doc](https://github.com/elixir-lang/ex_doc) from 0.19.3 to 0.20.2.
- [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@v0.19.3...v0.20.2)

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

mix.lock

@@ -6,7 +6,7 @@
   "ecto": {:hex, :ecto, "3.0.7", "44dda84ac6b17bbbdeb8ac5dfef08b7da253b37a453c34ab1a98de7f7e5fec7f", [:mix], [{:decimal, "~> 1.6", [hex: :decimal, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:poison, "~> 2.2 or ~> 3.0", [hex: :poison, repo: "hexpm", optional: true]}], "hexpm"},
   "ecto_sql": {:hex, :ecto_sql, "3.0.5", "7e44172b4f7aca4469f38d7f6a3da394dbf43a1bcf0ca975e958cb957becd74e", [:mix], [{:db_connection, "~> 2.0", [hex: :db_connection, repo: "hexpm", optional: false]}, {:ecto, "~> 3.0.6", [hex: :ecto, repo: "hexpm", optional: false]}, {:mariaex, "~> 0.9.1", [hex: :mariaex, repo: "hexpm", optional: true]}, {:postgrex, "~> 0.14.0", [hex: :postgrex, repo: "hexpm", optional: true]}, {:telemetry, "~> 0.3.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm"},
   "esqlite": {:hex, :esqlite, "0.2.1"},
-  "ex_doc": {:hex, :ex_doc, "0.19.3", "3c7b0f02851f5fc13b040e8e925051452e41248f685e40250d7e40b07b9f8c10", [:mix], [{:earmark, "~> 1.2", [hex: :earmark, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.10", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm"},
+  "ex_doc": {:hex, :ex_doc, "0.20.2", "1bd0dfb0304bade58beb77f20f21ee3558cc3c753743ae0ddbb0fd7ba2912331", [:mix], [{:earmark, "~> 1.3", [hex: :earmark, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.10", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm"},
   "jason": {:hex, :jason, "1.1.2", "b03dedea67a99223a2eaf9f1264ce37154564de899fd3d8b9a21b1a6fd64afe7", [:mix], [{:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm"},
   "makeup": {:hex, :makeup, "0.8.0", "9cf32aea71c7fe0a4b2e9246c2c4978f9070257e5c9ce6d4a28ec450a839b55f", [:mix], [{:nimble_parsec, "~> 0.5.0", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm"},
   "makeup_elixir": {:hex, :makeup_elixir, "0.13.0", "be7a477997dcac2e48a9d695ec730b2d22418292675c75aa2d34ba0909dcdeda", [:mix], [{:makeup, "~> 0.8", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm"},