Bump rdoc from 6.6.2 to 6.6.3.1 (#138)

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.6.2 to 6.6.3.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.6.2...v6.6.3.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -158,6 +158,8 @@ GEM
     nokogiri (1.16.2)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
+    nokogiri (1.16.2-x86_64-linux)
+      racc (~> 1.4)
     passenger (6.0.20)
       rack
       rake (>= 0.8.1)
@@ -223,7 +225,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rdoc (6.6.2)
+    rdoc (6.6.3.1)
       psych (>= 4.0.0)
     regexp_parser (2.9.0)
     reline (0.4.3)
@@ -272,7 +274,7 @@ GEM
       actionpack (>= 5.2)
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
-    stringio (3.1.1)
+    stringio (3.1.0)
     thor (1.3.1)
     tilt (2.3.0)
     timeout (0.4.1)