Bump sdoc from 2.2.0 to 2.3.1

Bumps [sdoc](https://github.com/zzak/sdoc) from 2.2.0 to 2.3.1.
- [Release notes](https://github.com/zzak/sdoc/releases)
- [Changelog](https://github.com/zzak/sdoc/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zzak/sdoc/commits)

---
updated-dependencies:
- dependency-name: sdoc
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Gemfile

@@ -34,7 +34,7 @@ gem 'turbolinks', '~> 5.2.1'
 # Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
 gem 'jbuilder', '~> 2.11.3'
 # bundle exec rake doc:rails generates the API under doc/api.
-gem 'sdoc', '~> 2.2.0',          group: :doc
+gem 'sdoc', '~> 2.3.1',          group: :doc
 
 gem 'web-console', '~> 4.2.0', group: :development
 gem 'listen', '~> 3.7.0', group: :development

Gemfile.lock

@@ -206,8 +206,6 @@ GEM
       omniauth (~> 2.0)
     orm_adapter (0.5.0)
     popper_js (2.9.3)
-    psych (4.0.3)
-      stringio
     public_suffix (4.0.6)
     puma (5.6.1)
       nio4r (~> 2.0)
@@ -254,8 +252,7 @@ GEM
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rdoc (6.4.0)
-      psych (>= 4.0.0)
+    rdoc (6.3.3)
     responders (3.0.1)
       actionpack (>= 5.0)
       railties (>= 5.0)
@@ -269,8 +266,8 @@ GEM
       sprockets (> 3.0)
       sprockets-rails
       tilt
-    sdoc (2.2.0)
-      rdoc (>= 5.0)
+    sdoc (2.3.1)
+      rdoc (>= 5.0, < 6.4.0)
     sprockets (4.0.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
@@ -279,7 +276,6 @@ GEM
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
     sqlite3 (1.4.2)
-    stringio (3.0.1)
     strscan (3.0.1)
     thor (1.2.1)
     tilt (2.0.10)
@@ -336,7 +332,7 @@ DEPENDENCIES
   rails-controller-testing
   rails-i18n (~> 7.0.1)
   sassc-rails (~> 2.1.2)
-  sdoc (~> 2.2.0)
+  sdoc (~> 2.3.1)
   sqlite3 (~> 1.4.1)
   turbolinks (~> 5.2.1)
   tzinfo-data