Fix internal docs generation

[vbaranov]

Motivation

An error occurs with mix docs comand:

Generated block_scout_web app
** (ArgumentError) expected a keyword list, but an entry in the list is not a two-element tuple with an atom as its first element, got: {{:init, 1}, [{:type, 67, :fun, [{:type, 67, :product, [{:user_type, 67, :opts, []}]}, {:user_type, 67, :opts, []}]}]}

Changelog

Update ex_doc dependency to the latest available version

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools